This project has hit a bit of a dead end but I appreciate your input a lot. I may get an opportunity to run tcpdump from within their network soon- which is what I was waiting for and why I didn't reply yet, but things aren't really happening.
My ISP gave me an rDNS and I was off several of those dumb blocklists within the hour. One person who could not previously connect to me now can, so that was the issue for that user at least. They were using Mullvad VPN, so Mullvad blocks based on uceprotect or a similar blocklist.