I would lean toward the credit union solution. They already exist and have roots throughout the area. In my opinion, that solution is preferable, rather than launching something new. I think that people have a stronger attachment to credit unions than they would to a new bank. However, the digital age is changing the banking experience.
If I am not mistaken, there are approximately 6,000 post offices. Would you have to install 6,000 reinforced concrete vaults and alarm systems in order to be able to keep lot of money? In any case, you already have vaults because stamps are valuable. I don't know if creating such spaces would be worth it. Perhaps a study was done indicating that this could be quite interesting and economically viable, but we don't know.
In fact, I think I would prefer a collaboration or synergy between existing businesses.