Except it's impossible nowadays, since most apps require Google Play Integrity attestation and doesn't work on GrapheneOS.

And if it wasn't for these apps (for example banking apps, since someone decided that it's now impossible to use your homebanking from a PC since it's not secure enough) I would already use a Nokia 3310.

Not true. I'm a customer of 3 top French online banks and of two top global brokerage cie and the apps work great. I have no issue whatsoever except with one health insurance app but their Web app is better anyway.

Only thing missing is contactless payment but there are European banks that support GrapheneOS without Google Pay so alternatives exist. And I can use Garmin pay with my watch (and offline and without Garmin Connect thanks to the FOSS GadgetBridge appp) if that was necessary.

I have no idea what you're talking about. I haven't yet run into an app that requires integrity attestation and doesn't work on Graphene. Even my bank works, although it flagged me as someone who probably stole my password, until I scanned my passport and face a few more times.

Loads of banking apps work fine on GrapheneOS, and the vast majority of non-banking apps require even less attestation and so work on GrapheneOS like on any other Android with Play Services present.