reverse_proxy is what you want then, but if you use an IP address as your listen address, Caddy won’t be able to have a TLS certificate issued for it - certificates are only valid for domains (with some exceptions that are not relevant here). If you were trying to connect to your server with https://192.168.80.131, that won’t work, you’ll need to do http://192.168.80.131 with that config, or proxy to AdGuard from a domain Caddy manages similarly to your mydomain.com example.
If you’re using a real domain (i.e. not a free dynamic DNS domain), I recommend using subdomains instead of subpaths for each of your services.
This wiki article may be relevant to explain the conundrum with proxying to subpaths: