agile_squirrel
I'm using a mm wave sensor for a similar purpose and it works well. I don't use it as an on trigger. I use it to help decide when to turn lights off.
For a cheap or free solution, you could detect when your TV is on (Chromecast integration, home kit TV integration, etc. or power monitoring smart plug) and don't turn off the lights if the TV is on.
The VPN part is tough to answer since it depends on the music service and VPN exit IP. You may find the VPN is fine if you use another exit node.
I've seen some interesting development with Music Assistant but I haven't personally used it. It's not sure if it can be commanded using the cast button.
I agree with starting with what you have to get learning, but I wouldn't run a Minecraft server on this machine. I'd start with everything else and then buy an old office machine to upgrade. It depends on the unit, but they are easy to upgrade basic upgrades like adding RAM, storage, and possibly a low power graphics card. Since most servers are always on, keep an eye on power consumption with a power meter.
Most carriers know your name, your internet usage metadata, and your location all the time using cell tower triangulation. There have been multiple reports of phone providers selling location data and sercurity breaches. If you have a phone that provides MAC address randomization and you use a VPN then I think using public WiFi is more private than having your mobile data turned on all the time.
I have my phone on airplane most of the time and use Wi-Fi with VoIP for calls and SMS. If I need mobile data, I have a Silent Link eSIM for mobile data only. If you ever need to make an emergency call you don't need a sim card or provider, emergency calls always work. While the main motivation is privacy, it's nice that it olny costs about $3 per month on average.
I don't know either. Swiftfin seemed more actively developed based on releases, but now it's great to have another active option. I haven't compared them but I am curious if anyone had.
Out of curiosity, why do you use this instead of Swiftfin?
What is the battery life?
As another solution, you could try Tuta Calendar. It's not self hosted, but is free for one calendar. You will be able to access their WebUI and they have clients you can download too.