I have a setup very similar to this. Although I’m fully on the Plasma Bigscreen bandwagon. But then again, I don’t have many custom services such as NFC login/custom background image logic like you do, and I doubt Plasma Bigscreen would play nice with that stuff. A couple of things I learned through my many attempts of setup:

• HDMI CEC doesn’t work on pretty much all x86 devices except for like some particular mini PCs, the Framework desktop, and the Steam Deck dock. Most ARM devices do though which is how I got around this by just using an Orange Pi 5. Another option is tunneling over DisplayPort if your device has a DisplayPort and you have a supported DP-HDMI adapter. Then finally, there’s also the PulseEight USB adapter if nothing else works.
• I’ve been using Waydroid for any application that doesn’t support Linux natively like YouTube or most streaming services (although I only use YouTube and Jellyfin for all of my content). You can use Waydroid Android TV which launches a near-native Android TV translation layer running Lineage OS which is de-googled and privacy protecting by default. You can launch the normal Android interface which has the entire Android TV UI, or you can enable multi window mode and just launch specific Android apps directly from whatever Linux launcher you use and they launch and work like normal Linux apps with TV controls. Very useful since Android has a much larger TV app ecosystem.