Thanks for the insight! I haven't had any subjects detected except "children" ever, but I only got an Intel Xpenology this summer, and before that I was using an ARM Synology which didn't support subject detection. So the timeline makes sense.
Anyway, I just downgraded my package to https://archive.synology.com/download/Package/spk/SynologyMoments/1.1.2-0511/ and am currently re-indexing, will update with my results!
Update: That did the trick! I now have 28 categories and it's still indexing