Before the subreddits went dark, I used a tool to see which subreddits I've posted to and commented on the most. Then, I added in a few subreddits that I had newly joined and so weren't represented in the data.
I had a list of 17 subreddits. I actually subscribe to over 30, but clearly the others weren't that important to me. I've replaced at least 7 of those (including the top 2) with Lemmy. Most of the others really need no replacement as they were just time killers.
About the only subreddit that I really care about that I haven't found a good Lemmy replacement for is r/LEGO. Yes, there's a Lemmy alternative and I've subscribed to it, but there are few people there.
So if I do return to Reddit, it will likely be for 1 subreddit only. I'll unsubscribe to everything else and deal with Reddit trying to push me into other discussions while I help the Lemmy LEGO community grow.