... I'm wondering if I really need to take up Mark's time toHe hasn't mentioned automating any file-based import from PG Offline archives. So that would probably involve at least as much of Mark's time as using the transfer agent. He has mentioned importing messages (only) from Mailman and other lists, but I think he has done those essentially as one-offs.
I've set up a trial site, and a site with a similar name that I haveI'm not quite sure what the question is here, You can transfer into either Groups.io group.
If you're going to switch the names around remember that deleting a group leaves its name reserved indefinitely. To avoid that rename the group first (the old name does not become reserved).
I hate to say that I would feel better if there was a fee for this,Others have mentioned upgrading your group to Premium. In addition to the Pricing page, have a look at the Upgrade page in your group (bottom most of the Admin buttons). Click the View/Change Plan button. There you'll see that one of the choices is "Group Transfer (Premium For One Month). If you're up for a voluntary payment that's a no-fuss way to do it.