This made me realise I’d copied over the incorrect certificate to the backend Caddy server. I had copied across /.local/share/caddy/pki/authorities/local/root.crt instead of /var/lib/caddy/.local/share/caddy/pki/authorities/local/root.crt.
@Rob789 The above paragraph in your wiki article under the heading Backends should be updated to use the absolute path to avoid any ambiguity:
root@caddy:~ # sudo HOME=/var/lib/caddy caddy trust
2021/05/10 13:40:41.376 WARN ca.local installing root certificate (you might be prompted for password) {"path": "storage:pki/authorities/local/root.crt"}
2021/05/10 21:40:41 Note: NSS support is not available on your platform
2021/05/10 21:40:41 define JAVA_HOME environment variable to use the Java trust
trust: trust not supported
After copying across the correct cert to the backend server, I then restarted both frontend and backend Caddy services and attempted to access the test site. Sadly still no go 
Frontend caddy log extract
{"level":"info","ts":"2021-05-10T21:41:44.437+0800","msg":"shutting down apps, then terminating","signal":"SIGTERM"}
{"level":"warn","ts":"2021-05-10T21:41:44.437+0800","msg":"exiting; byeee!! 👋","signal":"SIGTERM"}
{"level":"info","ts":"2021-05-10T21:41:44.443+0800","logger":"tls.cache.maintenance","msg":"stopped background certificate maintenance","cache":"0xc00035a690"}
{"level":"info","ts":"2021-05-10T21:41:44.444+0800","logger":"admin","msg":"stopped previous server","address":"tcp/localhost:2019"}
{"level":"info","ts":"2021-05-10T21:41:44.444+0800","msg":"shutdown complete","signal":"SIGTERM","exit_code":0}
{"level":"info","ts":1620654104.5705228,"msg":"using provided configuration","config_file":"/usr/local/www/Caddyfile","config_adapter":"caddyfile"}
{"level":"warn","ts":1620654104.6052015,"msg":"input is not formatted with 'caddy fmt'","adapter":"caddyfile","file":"/usr/local/www/Caddyfile","line":2}
{"level":"info","ts":"2021-05-10T21:41:44.613+0800","logger":"admin","msg":"admin endpoint started","address":"tcp/localhost:2019","enforce_origin":false,"origins":["127.0.0.1:2019","localhost:2019","[::1]:2019"]}
{"level":"info","ts":"2021-05-10T21:41:44.614+0800","logger":"tls.cache.maintenance","msg":"started background certificate maintenance","cache":"0xc00032ff80"}
{"level":"info","ts":"2021-05-10T21:41:44.644+0800","logger":"http","msg":"server is listening only on the HTTPS port but has no TLS connection policies; adding one to enable TLS","server_name":"srv0","https_port":443}
{"level":"info","ts":"2021-05-10T21:41:44.645+0800","logger":"http","msg":"enabling automatic HTTP->HTTPS redirects","server_name":"srv0"}
{"level":"warn","ts":"2021-05-10T21:41:57.225+0800","logger":"pki.ca.local","msg":"installing root certificate (you might be prompted for password)","path":"storage:pki/authorities/local/root.crt"}
2021/05/10 21:41:57 Note: NSS support is not available on your platform
2021/05/10 21:41:57 define JAVA_HOME environment variable to use the Java trust
{"level":"error","ts":"2021-05-10T21:41:57.226+0800","logger":"pki.ca.local","msg":"failed to install root certificate","error":"trust not supported","certificate_file":"storage:pki/authorities/local/root.crt"}
{"level":"info","ts":"2021-05-10T21:41:57.226+0800","logger":"http","msg":"enabling automatic TLS certificate management","domains":["www.xenografix.com.au","www.caffigoalkeeping.com","caffigoalkeeping.com","readymcgetty.com.au","www.udance.com.au","*.udance.com.au","caffigoalkeeping.com.au","udance.com.au","www.caffigoalkeeping.com.au","www.readymcgetty.com.au","xenografix.com.au","acme.lan"]}
{"level":"info","ts":"2021-05-10T21:41:57.226+0800","logger":"tls","msg":"cleaning storage unit","description":"FileStorage:/.local/share/caddy"}
{"level":"warn","ts":"2021-05-10T21:41:57.258+0800","logger":"tls","msg":"stapling OCSP","error":"no OCSP stapling for [acme.lan]: no OCSP server specified in certificate"}
{"level":"info","ts":"2021-05-10T21:41:57.259+0800","msg":"autosaved config (load with --resume flag)","file":"/.config/caddy/autosave.json"}
{"level":"info","ts":"2021-05-10T21:41:57.259+0800","msg":"serving initial configuration"}
Successfully started Caddy (pid=96255) - Caddy is running in the background
{"level":"info","ts":"2021-05-10T21:41:57.260+0800","logger":"tls","msg":"finished cleaning storage units"}
{"level":"error","ts":"2021-05-10T21:42:08.298+0800","logger":"http.log.error.log4","msg":"x509: certificate signed by unknown authority","request":{"remote_addr":"162.158.166.143:50592","proto":"HTTP/1.1","method":"GET","host":"test.udance.com.au","uri":"/","headers":{"Accept-Encoding":["gzip"],"Cf-Ipcountry":["AU"],"X-Forwarded-Proto":["https"],"User-Agent":["Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.93 Safari/537.36"],"Cookie":["tk_or=%22https%3A%2F%2Fheimdall.udance.com.au%2F%22; wordpress_logged_in_6f3be6329744d07b768d1565b796af6d=basil%7C1645523796%7Cp7aCGGrrNSGs9CR4PRXTX8cgzxAPWRW5H3EMiHsK5v1%7Ca678f9423db6329357457c5b072e5840131c33f05e2c861be9d697954217ae95; wp-settings-1=libraryContent%3Dbrowse%26editor%3Dtinymce%26hidetb%3D1%26editor_plain_text_paste_warning%3D1; wp-settings-time-1=1613987799; __cfduid=d73e22d7aa4142037dc86a5d6da76202d1618656636; tk_lr=%22https%3A%2F%2Fheimdall.udance.com.au%2F%22"],"Cf-Request-Id":["09f81c940700001aaca3904000000001"],"Cf-Ray":["64d396ccd9631aac-SIN"],"Cf-Visitor":["{\"scheme\":\"https\"}"],"Accept-Language":["en-AU,en-GB;q=0.9,en-US;q=0.8,en;q=0.7"],"Connection":["Keep-Alive"],"X-Forwarded-For":["106.69.185.51"],"Cache-Control":["max-age=0"],"Sec-Ch-Ua-Mobile":["?0"],"Upgrade-Insecure-Requests":["1"],"Sec-Fetch-Site":["same-origin"],"Sec-Fetch-Mode":["navigate"],"Sec-Fetch-User":["?1"],"Sec-Fetch-Dest":["document"],"Sec-Gpc":["1"],"Cdn-Loop":["cloudflare"],"Sec-Ch-Ua":["\" Not A;Brand\";v=\"99\", \"Chromium\";v=\"90\", \"Google Chrome\";v=\"90\""],"Accept":["text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9"],"Referer":["https://test.udance.com.au/"],"Cf-Connecting-Ip":["106.69.185.51"]},"tls":{"resumed":false,"version":772,"cipher_suite":4865,"proto":"","proto_mutual":true,"server_name":"test.udance.com.au"}},"duration":0.006602499,"status":502,"err_id":"nfeudwy0e","err_trace":"reverseproxy.statusError (reverseproxy.go:850)"}
{"level":"error","ts":"2021-05-10T21:42:11.888+0800","logger":"http.log.error.log4","msg":"x509: certificate signed by unknown authority","request":{"remote_addr":"162.158.165.126:18316","proto":"HTTP/1.1","method":"GET","host":"test.udance.com.au","uri":"/favicon.ico","headers":{"Accept":["image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8"],"Sec-Fetch-Mode":["no-cors"],"Sec-Fetch-Dest":["image"],"Referer":["https://test.udance.com.au/"],"Accept-Encoding":["gzip"],"X-Forwarded-For":["106.69.185.51"],"Cf-Visitor":["{\"scheme\":\"https\"}"],"Pragma":["no-cache"],"Sec-Gpc":["1"],"Cookie":["tk_or=%22https%3A%2F%2Fheimdall.udance.com.au%2F%22; wordpress_logged_in_6f3be6329744d07b768d1565b796af6d=basil%7C1645523796%7Cp7aCGGrrNSGs9CR4PRXTX8cgzxAPWRW5H3EMiHsK5v1%7Ca678f9423db6329357457c5b072e5840131c33f05e2c861be9d697954217ae95; wp-settings-1=libraryContent%3Dbrowse%26editor%3Dtinymce%26hidetb%3D1%26editor_plain_text_paste_warning%3D1; wp-settings-time-1=1613987799; __cfduid=d73e22d7aa4142037dc86a5d6da76202d1618656636; tk_lr=%22https%3A%2F%2Fheimdall.udance.com.au%2F%22; cf_ob_info=502:64d396ccd9631aac:SIN; cf_use_ob=0"],"Cdn-Loop":["cloudflare"],"Sec-Ch-Ua-Mobile":["?0"],"User-Agent":["Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.93 Safari/537.36"],"Sec-Fetch-Site":["same-origin"],"Cf-Connecting-Ip":["106.69.185.51"],"Cf-Ipcountry":["AU"],"Cf-Ray":["64d396e34aea1aac-SIN"],"X-Forwarded-Proto":["https"],"Cf-Request-Id":["09f81ca21200001aacac959000000001"],"Connection":["Keep-Alive"],"Cache-Control":["no-cache"],"Sec-Ch-Ua":["\" Not A;Brand\";v=\"99\", \"Chromium\";v=\"90\", \"Google Chrome\";v=\"90\""],"Accept-Language":["en-AU,en-GB;q=0.9,en-US;q=0.8,en;q=0.7"]},"tls":{"resumed":false,"version":772,"cipher_suite":4865,"proto":"","proto_mutual":true,"server_name":"test.udance.com.au"}},"duration":0.005877493,"status":502,"err_id":"0vkbgp8eh","err_trace":"reverseproxy.statusError (reverseproxy.go:850)"}
Backend Caddy log extract
{"level":"info","ts":"2021-05-10T21:41:50.878+0800","msg":"shutting down apps, then terminating","signal":"SIGTERM"}
{"level":"warn","ts":"2021-05-10T21:41:50.879+0800","msg":"exiting; byeee!! 👋","signal":"SIGTERM"}
{"level":"info","ts":"2021-05-10T21:41:50.887+0800","logger":"tls.cache.maintenance","msg":"stopped background certificate maintenance","cache":"0xc0003347e0"}
{"level":"info","ts":"2021-05-10T21:41:50.888+0800","logger":"admin","msg":"stopped previous server","address":"tcp/localhost:2019"}
{"level":"info","ts":"2021-05-10T21:41:50.888+0800","msg":"shutdown complete","signal":"SIGTERM","exit_code":0}
{"level":"info","ts":1620654111.0122516,"msg":"using provided configuration","config_file":"/usr/local/www/Caddyfile","config_adapter":"caddyfile"}
{"level":"warn","ts":1620654111.0166075,"msg":"input is not formatted with 'caddy fmt'","adapter":"caddyfile","file":"/usr/local/www/Caddyfile","line":2}
{"level":"info","ts":"2021-05-10T21:41:51.019+0800","logger":"admin","msg":"admin endpoint started","address":"tcp/localhost:2019","enforce_origin":false,"origins":["localhost:2019","[::1]:2019","127.0.0.1:2019"]}
{"level":"info","ts":"2021-05-10T21:41:51.019+0800","logger":"tls.cache.maintenance","msg":"started background certificate maintenance","cache":"0xc0002f2700"}
{"level":"info","ts":"2021-05-10T21:41:51.020+0800","logger":"http","msg":"server is listening only on the HTTPS port but has no TLS connection policies; adding one to enable TLS","server_name":"srv0","https_port":443}
{"level":"info","ts":"2021-05-10T21:41:51.020+0800","logger":"http","msg":"enabling automatic HTTP->HTTPS redirects","server_name":"srv0"}
{"level":"info","ts":"2021-05-10T21:41:51.021+0800","logger":"http","msg":"enabling automatic TLS certificate management","domains":["test.lan"]}
{"level":"info","ts":"2021-05-10T21:41:51.021+0800","logger":"tls","msg":"cleaning storage unit","description":"FileStorage:/.local/share/caddy"}
{"level":"info","ts":"2021-05-10T21:41:51.023+0800","logger":"tls","msg":"finished cleaning storage units"}
{"level":"warn","ts":"2021-05-10T21:41:51.045+0800","logger":"tls","msg":"stapling OCSP","error":"no OCSP stapling for [test.lan]: no OCSP server specified in certificate"}
{"level":"info","ts":"2021-05-10T21:41:51.045+0800","msg":"autosaved config (load with --resume flag)","file":"/.config/caddy/autosave.json"}
{"level":"info","ts":"2021-05-10T21:41:51.045+0800","msg":"serving initial configuration"}
Successfully started Caddy (pid=91239) - Caddy is running in the background
. Until Smallstep fully support FreeBSD, mTLS via Caddy isn’t available for the FreeBSD platform.
…sort of 
