That's what I do too (not iOS + GrapheneOS but the result is the same) as I was tired of fighting to make my bank apps and itsme (digital identity app in Belgium) work on my rooted phone.

Everytime I have to use a stock phone I'm appalled at the ads and I have absolutely no trust in any US or Chinese manufacturer. So I use them only for banking and digital id because that's presumably not what they actually care about.

It's not that expensive, I think many people have an old Android phone lying around, it doesn't have to be up to date.

It is very ironic that the solution is using an old, insecure phone full of unpatched holes for all important banking and id business, because that one is vendor-allowed while your state-of-the-art GrapheneOS is not.

My job's SSO moved to provider that either required an unrooted phone or a reliable Voice auth.

For 2 years the voice authentication worked fine (they call me, I type in a number) on my regular rooted phone. Then one random morning I just stopped getting the phone calls. "Network said no".

Complete lock out, nothing I could do except go out and panic-buy an unrooted phone not running Lineage and using a modern Android version. (I tried my older unofficial lineage phones without root, and no dice.)

I opted for a good phone I could postmarket later, but gosh did it set me back almost 1/5 of my monthly salary.