I don't know about FOSS stores but banks have been known to make apps and publish them on alternate stores (Samsung, Huawei, Amazon) if there are enough users. Off the top of my head, Revolut and ING Turkey have apps on the Huawei app store right now.

IMO this needs a regulatory push. If the EU encourages or requests EU-based app stores with relevant twists (regional for example), they will appear, and Google will have to accept them, and the banks will put their apps there.

No app developer wants to deal with Google Play, it's ass and a pain in the. If a company has a local audience (like a national bank) and they get a store that caters specifically to that country and it's easier to publish (and cheaper) than Google Play, they will use it.

Local stores don't have to accept all apps, either. They can be regulated stores that are only open to certain entities in that country like banks, government apps, telecom, utility, public transport etc. That would take care of the most immediate needs for their citizens and liberate those from Google's control.