Did you use the existing flarum_import.rb script? It’s hard to imagine that HTML is in the flarum p.content field (which is what goes into raw in the import script). I am pretty sure that flarum uses markdown, so I don’t know why you’d have HTML inraw. Or maybe the script is just that broken.
But strikethrough is what is in the HTML. Maybe you can just fix that with CSS.
Fixing the <POSTMENTION> is a bit more tricky since Discourse has quotes, but not mentions. The easy solution would be to just change those into a simple @${displayname} (and hope that the username is the same as before the import, or do a lookup in the user_custom_fields to find the updated username. Another thing you could do would be to include a link something like @mention said [here](/t/-/<discourse ID for topic 25>/<8>).
A crazy solution would be to write a script that pulls the markdown from the flarum database and updates the raw field to include that. It would still need to be cleaned up a bit (as for mentions and POSTMENTIONS), but it would fix a lot of stuff.
Another idea would be to freeze your site, do a mass deletion of all of the imported data, fix the import script, and run it again.
But you don’t have a lot of new posts since you moved, so maybe you could save them somehow, do a fresh import on an empty database, and then add those back. Dealing with new users would be a bit harder.
If you’ve got a budget, you can contact me or post in Marketplace.