Cool you should be ok then. You''ll need to specifically configure read-only routing, by the way. It's not as simple as just enabling the read-only attribute within the AG. (Sorry...

One limitation I've found is that several 3rd party backup tools (the ones that use VSS) can't back them up.

Absolutely. You have to look at the whole picture or you just end up "fixing" the same problem over and over again.

Where I am we try to meet the requirements of the developers wherever possible, so the ones that asked for SQL 2017 were put on an upgrade path straight away....

Also think about how you will schedule your native backups, if required.

Are you performing index maintenance on the databases? I would assume not but I thought I'd ask.

Er, no, sorry. You need a dev environment.

Is there a limit on concurrent snapshots?

The best thing you can do is just install Developer Edition in a test environment and try it.

Would a failover cluster (FCI) fulfil the brief for you?

The best thing to do is just try it in a test environment. Developer Edition is free.

Alternatively you could use Basic Availability Groups, which are essentially mirroring+, and...

If it wasn't a copy-only full backup (you can check this in the backup history) then it will have reset the differential bitmap. If you have all of the backup...

It's been a few years for me since I saw this, but you may wish to consider using trace flag 1448:

April 23, 2018 at 5:08 am