

You can but its dependent on the phone and USB-C adapter being used. Originally I was attempting to use 5G cellular as backup internet via a refurbished Samsung A54 5G. I was able to build the GRE tunnel between the phone and router, but the phone would either charge or provide ethernet, not both.

As a networking guy, for homelab setups the router is not core of your network. That role falls on the switch. In a perfect world, you’d have a layer-3 switch handling traffic between segments and only send traffic to the router for egressing the network or a few other cases. But in the real world, you have to start somewhere and that’s what you did. Don’t let anyone tell you that you did it wrong. If someone can’t make things work without having the perfect equipment, its probably the wrong hobby for those people.
Regarding network-wide adblocking, I had a squid proxy running that did this. Every machine was issued a self-signed certificate and the connections were basically MITM so I could check the calls being made. You can run into some issues with SSL-pinning in Android or things like HSTS for common websites sometimes, but overall it did function pretty well after tweaking.
If you do decide later to replace your existing router, I’d suggest trying to build your own. My current router is a mini-PC with dual NICs running Arch configured to do packet filtering, routing, a few automations, etc. It was refurbished and cost me about $80 USD. Its a really good experience in building servers and learning how various routing protocols work.