> for the Android case, as you use it from your bank's app, it would typically require some Google security assurances - so no Huawei phones allowed, for example

I don't know about Huawei, but actually most (all?) of the banking apps in Spain should work on a non-Google-certified Android builds. There's an community list tracking GrapheneOS compatibility at https://privsec.dev/posts/android/banking-applications-compa... and all of them currently appear supported just fine.