What is best and hopefully easiest way to set up Multistie?

I have been building several regional sites for a client over the last two years most with identical functionality using a specific library of forms per site and specified Roles.

I recently built a national store using a subdomain and used User Synchronization plugin to bring in specific Roles and individual User info to allow use by making store the Sub and regional sites the Masters. The regional sites have unique domains.

Would it be best to just forward these unique domains into a sub domain of store and use Multisite? I am trying to make change and User management easier.

Looking for thoughts on best way to proceed with this build.