GrapheneOS

All apps i've tried so far have worked so I would wager yes. I recommend check out the app obtainium and install whatever you can through it

Don't know, but f-droid's pipe-pipe works great

If it works on Android it will work on GrapheneOS.

Google Pay (specifically the tap to pay feature) is the one real exception.

I am very happy using NewPipe for YT/YTM, but i'll look into the revanced apps and see if they offer something better

I doubt there are many Grapheneos users running Revanced on their phones as it requires either root or microg, both are considered undesirable on grapheneos.

I read somewhere that microg doesn't work on grapheneos... dunno how true that is but it doesn't really matter as revanced uses its own version of microg that it installs itself if no root is detected. The version of microg that is installed by revanced does work on grapheneos.

Out of curiosity i installed Revanced Manager on my pixel 6a and gave it a go.
I got it to work with microg on both my main profile that doesn't use sandboxed google play and also in my private space profile that does have google play installed. One thing to remember, you need to use the same patched yt apk if installing on multiple profiles, not install revanced manager and patch the yt apk seperately as it will not install.

It worked fully on both profiles with the only difference i could find was that casting to my tizen tv would only work on the profile that has sandboxed google play installed.

Conclusion, after my tests I uninstalled it from my main profile as I only run foss software there. I think I will keep it in my private space profile which is basically my "anything else" profile where I have any needed, but non-foss crap (like banking apps).
I'm hoping that keeping it installed in that profile will allow me to quickly switch to it at times that Tubular doesn't want to work or the odd occasion I want to cast it to a tv.