I have a Samsung Galaxy J3 (2018) smartphone which currently has the stock Samsung Android OS installed on it. I wanted to install an Android "distro" that doesn't spy on me, like Graphene OS, but I couldn't find a ROM for it. Since I would probably need to compile AOSP from source code anyways, I though, why not install Gentoo on my smartphone (doing the compilation on a more powerful computer using distcc). I have already installed Gentoo on both my laptop and desktop from a stage3 tarball and I'm loving it, so I guess doing the same on my smartphone wouldn't be too hard.
Now, the problem is that I need to use a few apps that are not available on Linux, like the proprietary app that I use to pay for my bus tickets. How well does waydroid work?
As others are saying, that's not really an option unless you're really dedicated. IF it has an unlockable bootloader you could technically get to compiling and tinkering to get everything built, but in order to get a phone all set you'd need to get the right drivers and do a whole lot of tinkering (like full time job levels of building and tinkering) kind of deal to get it built. Phone's aren't so plug and play like computers.
If you there's no rom support and/or a permanently locked bootloader but you want an OS without x y and z you can always try to fire up ADB bridge and disable stuff. You could also accomplish the same by rooting, though it's a bit of a security risk (though not as overblown as some people say IMO).