I agree, but unfortunately it does cost money (way more than you think) to host something online, even a small Lemmy instance. The more traffic you have, the more it costs. The same goes for time spent on admin, which shouldn’t be free unless it’s a passion project.
Linux is not like a custom ROM, it’s just an OS like any other (but many distros do respect your privacy). But yeah, custom Roms are the only way to have privacy on phones other than just using dumb phones. Support open source projects and manufacturers that make devices that use open source software!