To multisite or not to multisite…

The problem I see right now is that my users will have to create separate accounts for each of these sites and that doesn’t seem user-friendly. This is the ONLY goal I’m looking to accomplish by using multisite. From what I’ve read, migrating these sites into a WP multisite would allow them to log in once.

Not if I understand correctly. Multi-site does share users so each user only needs one set of login credentials for use at any user-sharing site — all sites’ tables must be in the same database — but multi-site still requires a separate login at each site since the shared user tables in the database contain site-specific permissions for each user at each site. I have no experience with multi-site, but I have used user sharing outside of multi-site and it worked the same as in a multi-site setup.

…they would STILL have to login on each subdomain site with the same login credentials. right?

Yes, that is my understanding…and I do not know the other advantages or disadvantages of mult-site well enough to offer an opinion there. What I do know, however, is that you would still need the tables for all sites to be in the same database for user sharing — each site would still have its own SQL user assigned to that single database — even while the remainder of each WordPress installation is separate.