Is using a cheap Android device (the cheapest Android phones are less than $100 on Amazon) an option? The idea is to use that phone for 2FA or whatever is app is necessary for, and use a degoogled device for your other day-to-day activities. It's not ideal because you need to spend some extra money, but it buys you a lot of privacy.

Does the web app for the bank actually selectively block mobile phones? I just checked and Chase here in the US lets me log in on Brave Mobile on iOS. Perhaps your bank lets you log on in the browser.

Have yoz really checked for GrapheneOS? Which country?