I agree with you here. In itself best would be to not use online banking at all. As I said, this is hypothetical and doesn't apply to my personal situation (anymore). I just realized it the first time after degoogling and seeing this being a struggle many have when transitioning to a degoogled device, it kept me thinking.

Nevertheless, using a banking app is very quick and convenient and is therefore used by many people. I don't see privacy education going that far, that people would abandon this convenience. Therefore I was wondering if an open source application would be possible here and bring the benefits that OSS usually offers in other circumstances.