Well, we took the plunge and did the scary (for us anyway) upgrade from pictrs 0.3.x to 0.4.x. All went semi-smoothly, we had no images for a hot minute until we worked out there had been a previous failed migration that left over some remnants.
For anyone in the same boat - deleting the sled-repo folder will force another migration from 0.3.x to 0.4.x. I had a lot of trouble finding their git repo as well, eventually we found it along with some steps on how to migrate from 0.3.x to 0.4.x here: https://git.asonix.dog/asonix/pict-rs#user-content-0-3-to-0-4-migration-guide
During this time we've also upgraded from Lemmy 0.18.0 to 0.18.1.
Happy days!