The other fella covered the more general user-generated approach, but the WefWef app has a way to migrate from Apollo using the JSON export tool they (Apollo) provide. Looks like the grab the JSON dump, parse out the subs, then generate a big list of community search links in-app.
Expanding on that, a potentially good idea to make this as easy as possible is to find a way of having the user export a list of subs from their Reddit account (either by biting the bullet and using the API or developing a user script or browser extension). Allow clients to register an anonymous user ID (to avoid tying identities together too hard) with such a list. Then the clients can update this user with what communities they join via what instances, along with what instances they joined at all.
Then your service would feed them recommendations."Users from /r/programming[,...] tend to join [email protected]" and/or "Reddit users like you usually join the fediverse through programming.dev".
It may be worth DMing some of the Lemmy client developers to see if they'd be interested in such a service or if they have any better ideas. Smart people, them.
If you do end up doing work on this, please do post any cool ideas you have! It's a neat domain space.
Hope you have a great day, good luck!