I don't know if it will have any effect, but if you haven't tried bridges, you can try under Settings -> Connection -> Use bridges ON -> Select a bulit-in bridge
CloudFlare only sees your exit node and thus the bridge shouldn’t have any effects. In fact, bridges should be used exclusively for circumventing censorship.
Well, there are other solutions. But I wonder, if there will be enough users, willing to switch to P2P-based forum. Because then, DDoS-protection would work as in Bitcoin: if you have many nodes, with proper setup, then you cannot DDoS some P2P network that easily. However, will people agree to run their own full nodes, and download all posts, no matter if they are going to read them or not? I don't think so, and that's one of the reasons, why we have so many centralized points on the Internet: not every user will agree to share its own resources, and set up a full node.
By the way: I posted this message from Tor, so it works for me. However, I encountered similar problems in the past.
It would probably be better to just pay more rather than to develop it again from the ground up. Hosting the forum on onion can potentially be a good idea as well. CloudFlare provides their services on onion sites as well and being able to segment Tor and normal users would be better.