You can check for compatible banking apps here: Banking Applications Compatibility with GrapheneOS | PrivSec - A practical approach to Privacy and Security
It’s an unofficial community-ran resource but it’s updated pretty often.
I know some banks allow you to receive many notifications over email or text, maybe check if your banks support this feature?
I’ve also learned that iOS is susceptible to VPN leaks, meaning some of your data may be transmitted outside of your VPN. Apparently the issue has been known about for five years and Apple have done nothing about it. It’s entirely unacceptable, and entirely excludes Apple products from my consideration.
Thanks I appreciate that. 
I’ve decided.
Going with GrapheneOS. Thank you all for your help. I really appreciate it.