How should I manage multiple service accounts without key files?

[Ezard]

What's the best way of managing service accounts for multiple different projects?
I have ~10 different projects which are all linked to different Google Cloud/Firebase projects (different projects for different clients), and I'm looking for a nice way to manage this
Ideally I'd not have to download any service account key files, and I'd use something like service account impersonation; however, I can't find any docs on how to do this, so any advice would be appreciated