I’m glad to see you are working on this - what’s the latest news on it? I’d be glad to contribute as a tester or to bounce ideas for what this should be able to do.
I’m not sure what JSON and TXT formats look like, but in the past I have always appreciated any forum tools that allow export into MBOX format, which can then be imported easily into a mail client.
Back in the day, I had some success with a forum called FUD Forum (great name, I know) that had a very slick email integration. It can be fed MBOX files to import email discussions. It automagically generate the users if they don’t already exist. I used it to import mailman archives.
FUD Forum ended up not working well as an actual forum, though, so I gave up on it and moved to Drupal organic groups which ended up being even worse. Maybe Discourse will be the answer for me in this new era.
Here’s some ancient history from FUD Forum in case you are interested.