parbleu
Well, a separate user profile is going to be better than just private space.
I believe the most ideal setup would be to install full Google Play Services (burner or real account, user discretion) in that profile. Then downloading your banking apps within that profile.
If the banking apps require Play Services, I don't think they're going to be happy being pushed to a profile that doesn't have that running despite being downloaded in a profile that had it.
Aside, keep in mind you still may need to adjust exploit protection for them.
Further reading
https://privsec.dev/posts/android/banking-applications-compatibility-with-grapheneos/