I've marked below if its a GOS issue (most are not).
(NOT A GOS ISSUE) :: Tap to pay can work on GOS, Curve and some banks payment systems work, the issue isn't GOS, its Google.
Granted however, if you cant use a service like Curve, tap to pay is basically a no go, but it does work, just not google wallet.
(NOT A GOS ISSUE) :: I've never had an issue with a banking app (UK), I have Monzo currently and it displays a warning saying the phone maybe rooted, but it works fine for me, I've used First Direct, BarclayCard, Virgin etc and never had an issue, but sometime banks do not work unfortunately, see if the bank has a usable web app?
Find hub doesn't work, haven't ever got uTag working either which is a shame, dont think there is a fix for Find hub given its reliance on GMS and being installed as a system app.
Both of the above are due to Google not approving GOS to be white listed for the Play integrity checks (its beyond the CMS checks), Google wont approve it either, given GOS wont preload GMS / GPS as system apps.
(NOT REALLY AN ISSUE) :: Apps can notify in the background, Signal for example requires turning off battery optimisation (allows it to run in the background all the time), which causes some minor battery drain (very minor in my experience, not noticable), but yes, notifications work with Google Play Services..
Unowned airtags via Find My requires GMS again, same as Find Hub.
(NOT A GOS ISSUE) :: Bluetooth disconnects that is by design, its essentially a different phone is a different profile, not much you can do here.
(NOT A GOS ISSUE ) :: Google watches with 4G wont activate their 4G connections using GOS, there are work arounds however.