If you really want to keep things local without paying any fees, you could also use Smallstep (https://smallstep.com/) to issue certificates for your services. This way you only need to add one CA to your trust store on your devices, and the certificates still renew periodically and satisfy the requirements for TLS.

I suggested using a domain given they already have Caddy set up and it's inexpensive to acquire a cheap domain. It's also less of a headache in my experience.