KDE Connect as another uas suggested.

Only buggy issue I have is that I use a VPN on my phone to access my local network when away from home. I have the VPN set up with Hairpinning so I don’t have to disconnect from the VPN while at home which occasionally causes issues on my steam deck as the version of KDE Connect on the steam deck does not appear to have an easy way to connect to device via IP. On my desktop and laptop I run Pop!_OS and use GS Connect which gives me an option to connect to device via IP so I have less issues with my phone just randomly not appearing on those computers.

Back in the day I ran a pfsense VM on ESXi before broadcom bought them. I found it easier to just purchase a cheap Unifi AP and spin up a VM for the Unifi controller app (I’m assuming you can still run that on your own hardware, its been a few years). I think I got a UAP 6 lite or something like that. If I recall it was about $130 CAD with the PoE injector.

Techincally its not ideal to run your firewall on the same device as other things, but I never had any issues.

Also, does the mini PC have multiple NICs? You will have a bad time if it does not.