PSA: You should know that Settings > Migrate Subreddits exists. All you do is login to your old Reddit account, copy a “multireddit” link, paste it in Voyager and you’ll be able to search the fediverse for those topics/communities to subscribe to.
You may find communities that are missing here if you’d like to run/manage your very own community on a topic you enjoy.
Hope this helps. I just found this feature today and subscribed to a bunch of communities.
In this app (that this community is for and you’re presumably using), go to Settings, then Migrate Subreddits and press on the part that says ‘Multireddit link’ until the paste option comes up.