I think I have some experience with installing banking apps in eOS (see my HowTo).
I have installed my DKB Tan2Go app on 3 different devices with eOS nougat and pie. It even runs without the Magisk Hack, as long as the device is not rooted.
But I can’t get the app to run in eOS GSI pie. Unfortunately SU is installed by default. I have removed several files from /system/bin, tried to remove the rest with the SuperSu remover, but without success. The app still refuses to run.
I don’t know why root is installed by default. I am absolutely against rooting, because most people don’t need it.
The ‘magisK hack’ is also not working, because of the existing SU.
The question now is, how do I get a ‘clean’ system where banking apps work as well?