How can I set up 1Password to use my Phone PIN instead of the full password every time I log in?

Hi @KatyThom, thanks for your question.

1Password 8 for Android can be unlocked with an account password or biometrics, not a PIN.

As you mentioned Biometric unlock isn't a good option for you, another would be to extend the Auto-Lock on Exit interval from Settings > Security. After extending that interval you can open 1Password with your account password and leave it running in the background. Until that interval ends, 1Password is locked, closed completely or your device restarts, you can Autofill and reopen the app without your password.

I've also gone ahead and added your interest in a PIN unlock option to our internal feature request tracker.