If we want FOSS banking apps, I think the first and most important step would be legally requiring banks to provide standard APIs.
Germany supposedly has an open standard banking API. I don’t know if it’s legally mandated but in principle its mere existence and acceptance by some banks would theoretically be sufficient to inspire FOSS apps. I vaguely recall that GNU Cash recognizes that standard… can anyone confirm?
I don’t think I’ve seen any portable FOSS banking apps for any country in the F-Droid official repos. Which suggests that a standard open API may not be sufficient. Or perhaps I have something wrong here.