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.
That's the one I joined. Only 19 people are subscribed there, though, versus over a million in the LEGO subreddit. It's got some room to grow before it can be a complete replacement for me, but I'll definitely be sticking around there.
It is https://redditmetis/. You put in your username and it will tell you stats about where you post and comment among other things. (Look for Top Subreddits By Number of Comments and Top Subreddits By Number of Submissions.)
Yes, that worked - although not on my phone for some reason. Or rather, it opened on my phone, but crashed when I tried to get it to process my username. It worked correctly on my desktop, though.
Can't say I like it much. It knows too much about me in some areas, and it's completely wrong about me in others!