I call it sequel but I've heard squirrel and squeal.
Tigwyk
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.
And every time I try to pull off a sweet gymnastics maneuver below an air conditioning unit suspended by failing screws...
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.
I miss lockpicking, it's so cathartic. I used to have a small set of picks and folks near my desk at the office would often try to pop a padlock I kept around when we were bored. I liked how everyone seemed so interested in the ease with which you can pop many locks.
If you don't mind a copy of their "Lite" version that often ships with audio hardware, you can google a method to generate a valid license key for that version that doesn't require buying anything. ;)