Where you matched requests by path prefix in Caddy 1, path matching is now exact by default in Caddy 2. If you want to match a prefix like /foo/ , you’ll need /foo/* in Caddy 2.
This means that the / in your php_fastcgi directive will only match requests to / and nothing else. Just remove the / and it will work. The rewrite is also not necessary because that’s now built into the php_fastcgi logic, which does a try_files rewrite similarly to what you had in your v1 config.