@meowki It would be great if banking apps could work without Google Play Services; that said, keep in mind that on GrapheneOS, you install Play Services and Google Play as standard, non-privileged apps that run in the hardened sandbox.
This is a significant difference compared to stock Android, where Google Play Services runs as a system app with elevated privileges that you cannot control. MicroG works in the same way and is often mistakenly presented as a more private alternative to Google Play Services.
What cross-app sandboxing doesn't protect is communication between apps based on mutual consent. If you install Instagram and Facebook on the same profile, the apps still only have access to what you authorize them to access, but since they belong to Meta, they could exchange telemetry data with each other.
To stop this, the solution is to use a system-wide secondary profile, which offers excellent isolation but is somewhat cumbersome to use, or the private space, which provides less robust isolation but is easier to use. This decision really depends on your threat model and whether or not you consider plausible communication between these applications to be acceptable.
https://grapheneos.org/usage#sandboxed-google-play