Looking for best way to import members from existing site

I feel like Membership 2 Pro is the solution for this but really looking for the best way to import users from an existing site and manage members moving forward. Current site is a local newspaper with about 7K users, registered and approved to comment, once they're logged in. May choose to setup a paywall in the future, but not now.

All "members" are free, just really registered site users over the years. Export of existing users has a few fields that are not yet in the new site (gender, date joined, last updated, etc.). When trying to import using WP Ultimate CSV Importer we have the ability to assign to custom fields, but haven't been created yet and not sure if we'll be able to access them from WP once they're in.

Looked at BuddyPress, which seems like a decent option, but it looks like it leans heavily toward setting up an internal social network, which isn't a functionality we're looking to roll out yet.

Is it as simple as setting up Custom Fields, importing as users and sending out invites to visit the new site and update their password? Not sure if we're putting too much (or not enough) thought into the process.

Really just reaching out to you guys to leverage your years of experience with WP - What's the best way to get users from an existing site migrated into a new site? To clarify, none of these "users" will have permissions beyond commenting on articles once logged in, unless we assign/update those privileges later.

Sorry for the rambling, we've just looked at so many plugins and blogs for answers and it just seems to get more confusing. Thinking we should go with the easiest option for the short-term (initial import) then look to the best option to manage in the future.

Would appreciate any feedback at all. Thanks for your time!