m-s is it much safer to install the version of the banking app from google play store compared to say, aurora?
Objectively, Google Play Store has more checks to make sure you're downloading an app from Play Store. However, unless you're the target of some sophisticated hacker group because you're rich or influential, I don't think it really matters much. I personally prefer to play it safe anyway and use Google Play when I want to be certain.
m-s and, with respect to what malicious things other apps might do to a banking app, is the app safer running in sandboxed GPS or on a fully de-googled user profile?
All apps are sandboxed and apps don't run in a sandboxed Google Play Services. The difference is whether they're in the same profile or not. If you're asking "safe" as in "security," then I don't think there's a difference. Malicious apps are sandboxed too, so the same protections are in place regardless of the profile they're running from.