You can use Screen Time to prevent yourself from installing new apps. First, delete all applications aside from your banking apps. Next, go to Settings - Screen Time - Content and Privacy Restrictions - iTunes & App Store Purchases. From here, you can disable installing apps, which hides the App Store on your phone. You can also disable Safari from Screen Time. If you find yourself turning off Screen Time, have somebody else set a four digit passcode for you. You'll be left with a phone that can only text and bank.
Thanks for the suggestion re:screen time - I have attempted this multiple times, and even got my partner to set the passcode. However I just bypass it and reset it via forgot passcode and my Apple ID password
Perhaps you could have your partner set your Apple ID password? Or you could change it to a very long string that is a pain to enter, write it down, and store it in a place where it is mildly inconvenient to access.
