The part about changing the login screen seems to be not entirely true. There is also this tool that claims to be able to generate rpm from sddm themes that you then can layer onto your system image. Take this with a grain of salt though, as I haven’t tried either method because I honestly don’t care how my login screen looks.
I have a Fairphone 4 since release. There where some bugs for the first couple of months. Now everything works as it should and I have no complaints regarding day to day usage. I am not using my phone for much more than messaging, navigation and some light web browsing though, so my experience might not be representative.