GrapheneOS would be good for separating identities because of the profiles feature. I don’t see a way to robustly achieve your goals on iOS.
Profiles are ideal because you can change your behavior based on the profile and they include customizable colors which you can mentally model to easily know what kind of activity is being done in that profile.
The profiles have separate VPN slots which there’s no equivalent feature for on iOS and I’m not sure there’s any way you could separate IPs on iOS besides using browser methods like Onion Browser.
On iOS, apps can see each other which could cause a problem and they have permanent identifiers.
On GrapheneOS, you can use profiles or private spaces (apparently they are the same thing under the hood) to prevent inter-profile-communication and apps listing other apps.
The inability to bank
Have you checked your banking apps specifically? Over 90% work.
I heard that a main Twitter dev said they would roll out a fix for this and there’s an app overhaul announced so that could contain the fix.
I’ve also found PWAs are much better on GrapheneOS than on iOS. PWAs would be better for you anyway. There’s no problems with PWAs with Twitter or Instagram and you wouldn’t have to install Google Play Services or worry about compatibility. It seems Instagram works fine, though.