Bitwarden, Aegis (2FA app for Android), Syncthing are probably the most impactful

Pixel 4a models do have headphone jack if you don’t need the newest models, unfortunately no sd card slot still

Don’t know that kind of site but i would bet Google Pixel phones have one of the best support from custom roms including Lineage, Murena, Calyx, Graphene and whatnot

Still enough to generate heat for something like dust to catch fire if it shorts