Help me adhere to best practices. Please!

You need to distinguish corporate intranet (durable/permanent) from workgroups and projects (ephemeral). Communication and hub sites are for corporate intranet - it's where employees go to find HR policies or order a new laptop from IT. Microsoft Teams is for projects and people.

I would set up a communication site (aka public publishing site) as the hub for each "Family" in  your organization - IT, HR, and each area of practice. This gives each group a place to put information aimed at the entire firm about "who we are and what we do". Make sure you establish navigation across the hubs so people can get from one "family" of content to another.

Then you move to Microsoft Teams. I would have the lead attys for each case set up a Team for that. This will give them an O365 group for Planner, Stream, etc. As they add people to the case, those people will be able to see each case in Teams when they open it. They can set up channels for each aspect or topic of a case. People can then choose to follow/not follow a channel based on whether they are involved in that aspect of the case. This goes a long way towards cutting down on email notifications for stuff you don't care about. Teams can be archived when the case is finished. These sites do not HAVE to be attached to a hub site, but they may choose to. 

IT, HR, etc. will need a workspace and they may be OK with Teams or regular team template sites with email. I would let them choose.