Well, I got that bandwidth thing sorted
Boy howdy that was a time. Short version is someone was running a discord bot that scraped RSS feeds and... it went a bit haywire and was basically pulling down all the HTML files from Zine of Millie every time it ran. I let the person running it know and it should be sorted now. So I've taken down the neocities mirror and put Zine of Millie back live on my own hosting, since we should be good now.
Meanwhile I've learnt important things like: where my webhost keeps its log files, so if I see something like this happen again I should be able to be more proactive and just block an IP or two if necessary.
Appreciate the advice from all my more technically-minded friends. I am just glad it's not like some AI bot or something.