I finally found the script, sorry for the massive delay.

https://github.com/Fmstrat/lcs

It’s named “Lemmy Community Seeder” which is probably why I couldn’t find it anywhere, I wasn’t searching for those terms.

I can’t seem to find it in a Google search now so I’ll take a look at my server in case I ran the script there and saved a copy. There appear to be a lot of similar tools now to assist with people moving over from Reddit but this script was really quick and handy.

I did this on my instance. You create a new user and give the script those credentials, it goes out and subs to all the trending communities across the various instances so now my instance has a big mish-mash of communities federated, not just the ones I originally subscribed to on my personal user.