Can you not use a browser to access banking services? If not, you should vehemently complain and/or move to another bank. And while cash is better for privacy, you can always use a credit card instead of your phone.
If nothing else works, my suggestion would then be to buy a cheap Android phone just for that indispensable banking app. Just leave it home in a drawer powered off when not in use, and use your a degoogled phone for your daily business.
