So, I use a FP4 with /e/OS and I like it.

First things first: some things will break. Not many, and not often, but it happens. Mostly Google stuff, but on Android a lot of stuff is Google stuff.

Recently GMaps wasn't working for a little longer than a week. I was still able to use HERE WeGo (my current favorite) and others even with Android Auto, so it was no problem for me, but still.

Banking apps and such almost always work, but there is a non-zero chance that one of those will break, even for a short while. I have three banking apps and they work flawlessly, plus itsme (Belgian gov app) and a German health insurance (this one refuses to login with fingerprint, but pass works).

Android Auto works, but I don't think I ever managed to get Chromecast to do anything.

You do get something in exchange. The privacy improvements are there, and the OS-level adblocking as well.

But you have to accept that occasionally there will be a nonzero level of discomfort.

You could keep the old phone around for the apps that don't behave, or you could use the old phone to test /e/OS before ordering.