Have you taken any backups since changing the recovery model?

That's not just a little shady, it's downright wrong.

I found Brent Ozar's blog invaluable when I was starting out (actually, I still do). They used to offer a 6 month DBA training plan pdf that was very helpful....

Should there be more files? Please give us the exact steps you are following when you get that error message. Are you restoring/attaching/doing something else?

You haven't got any antivirus scans running, have you? .baks would usually be excluded but it's worth checking.

I ask our Windows team for that; it depends how things operate where you work.

The folder used for data synchronisation can be on either of the servers, yes. You don't need a third one.

For the service account you would ideally use a...

There isn't a fixed number because it depends on what resources you have available.This is a good article about it: https://www.red-gate.com/simple-talk/sql/database-administration/alwayson-availability-groups-what-not-to-do-when-adding-databases/

It could still be a Kerberos issue. Are you using the same account across all the servers? (I warn you now, my SSIS skills suck so I might not get...

Hi- that's normal (i.e. not a duplicate).

I always use a gMSA for my SQL instances even if they're stand-alone because if it turns out we want to add them to a cluster later on it's much...

We don't do it here (a fairly large estate) even though it sounds like a good idea, because we grant access to certain specific file systems to our gMSAs, and...

You could run them all under the same gMSA. I wouldn't do it myself because I like to have distinct accounts for each instance (or group of instances where Availability...