which kind of kills half the point of using GrapheneOS in the first place

Absolutely not. Google Play services are much less invasive on GrapheneOS compared to other ROMs or the stock OS, since they run in the normal Android app sandbox, just like any other app you install. You can control all permissions, and uninstall them at any time. They do not get any special privileges, as it would be the case when running stock Android. You can also confine Play services in a separate user profile or in a work profile through an app like Shelter (user profiles offer better isolation).

If you’re not (e.g. you install via Aurora), then it’s incredibly unlikely your bank will work.

As I said, it highly depends on your specific bank. My bank in Germany works totally fine on GrapheneOS without Play Services. YMMV. That's why I linked to that list.