I can understand your perspective, thankfully with things like Obtainium centralising installs from vatious sources for FOSS apps really helps. 

Banking apps 99% of the time require Play Services as a minimum as do reliable push notifications as any apps from the Play Store either directly or from Aurora Store frontend will depend on FCM (Firebase Cloud Messaging from Google).

FOSS apps using web sockets or another implementation they'll require being exempted from battery optimisation.

Also on the camera side Pixel Camera and Google Photos work together flawlessly for all Pixel related features alongside sandboxed Play Services but again if your preference is for iOS over Pixel methods there wouldn't be anything GrapheneOS could do.

Glad you gave it a go and grateful for you taking the time to reply. Take care.