I've been the victim of similar abuse before, for my mail servers and one of my community forums that I used to run. It's frustrating when you try to do everything right but you're at the mercy of a cold and uncompromising rules engine.
In the ticket I just opened (see sibling thread), I asked which blocklist my domain was on. Maybe let's see what comes out of it, perhaps they can improve the process (e.g. drop that blocklist, or notify the abuse record of domains which they're blocking so that domain owners are at least aware of where they can go to fix things)
I don't see contact info on your profile or website/blog, but I can post here what the outcome is
You just convinced me to ditch quad9.