I do use Lineage base OSes and I'm really thanksfull to its contributors allowing me to extend the life of my devices for years, but there is still a tradeoff in term of security, the firmware blobs which are closed source cannot be updated so those do not receive any security update. Unfortunately, there's not much that can be done about that without forcing manufacturer to open source those blobs or maintain their hardware for longer periods of time.

Yeah supposedly, but I wasn't able to buy a used Pixel 3 that wasn't boot locked. I tried 4 of them, all were locked by the carrier.

Should be illegal imo to stop supporting a phone that's also locked. All it does is generate waste.