There was a tool being promoted a couple of days ago that sounded like it would help port your Reddit account over to lemmy by (I’m assuming) looking for identically named communities and subscribing to them. I went scorched earth on Reddit and deleted my comments and accounts, so I didn’t look at it beyond thinking it was a good idea.
I wish I could remember what it was called, but knowing that it’s out there might help you to track it down.