Stephen, wow a wealth of information there! Thanks! To answer / explain a few things in more detail:
1: GDPR - we don't archive any lists, period, ever. Once an email goes through, it's done. So that shouldn't be a big issue. (that also takes care of the redact issue you mentioned) We are as compliant in all respects as possible, and we have had little issue with anyone, maybe 10-15 requests for GDPR level removal since it came out. It seems to me that Mailman 3 is the way to go, perhaps it will last as long as 2 did, so one less upgrade to do.
2: FWIW - we're trying very hard to REDUCE effort, and reduce costs. We're getting off a mixed Drupal 6 /8 and vBulletin environment for just that reason. That's ALSO part of the reason for the move. And we've been running this for 25 years, and I'm technically well past retirement age, so I'm looking to have some time for me. Time to get out and ride my motorcycles more. :)
3: Batch script to mass convert - not me! I suspect I COULD - but the time involved is no way worth it to me.
4: MySQL is available for as many db's as needed, so that's fine.
Hosting the lists is a thought, but again, trying to reduce costs, and since my server already HAS mailman 2, that seems unnecessary.
So, it comes down to, in my mind: A script to convert my ezmlm lists to EITHER Mailman 2 or 3 If 2, there is apparently a script to upgrade to 3 anyway, and cPanel will eventually upgrade to 3. If 3, need to install 3 on my server (VPS) - not sure how involved that is, but should be doable.
I did get a couple of DM's re hosting a list, one even suggested they can do the conversion, so will check out if they will do the conversion for a reasonable fee.
thanks for your help, and to anyone else who has ideas!