Security isn't binary, it's a scale. "Even if" you install all apps in the owner profile, you're still more secure using GrapheneOS than standard Android/AOSP. Assuming ease of use is important for you, I recommend to:
- download apps from official sources only
- give apps minimum permissions
- enable exploit protection features (disable for some apps)
- enable bank specific security features (2FA, pin, etc.)
If you're worried about someone grabbing your unlocked phone, I recommend to setup a secondary user profile or private space where you have your bank app (and other sensitive apps). This way these apps can be secured with a different pin/passphrase.