It seems the attack on my server is fading away. I haven't been paying much attention since invoking "HTTPS always" mitigated the POST overloading. The traffic has long been waning from its thousands of visitors and tens of thousands of hits, to its recently low levels, with another drop in the last week or so to hundreds of visitors making thousands of requests per day.
Soon I expect it to have died completely. Still weird that it happened at all, and I still wonder why.