Понизить небольшую юридическую фирму от DC до самбы или рабочей группы?

I would continue to use Active Directory in this scenario.

Can you elaborate on the pain-points of the current setup? What seems complex and what are the user-felt experiences with the setup?

I have 8-user Active Directory installations, and they benefit from the central auth/management and policies like folder redirection. There's no downside to this type of setup, as there's a deep pipeline of Windows engineers who can help establish the initial configuration properly.

I'd reach out to a consultant to help develop a migration path. This doesn't appear to be a complex situation.