How demanding is it to make an onion version of the forum? Considering that these attacks last longer and longer, it would be really good to have an onion version that, if I understood correctly, would be resistant to such attacks.
Not very. Onion sites are vulnerable to DDOS in a similar fashion by exhausting its resources.
Again, I don't think that theymos will buy the idea because of the reputation of the forum. Having an onion version might take the government's eyes to the forum and some kind of legal compliances would be required.
It is not illegal, to my knowledge for sites to host onion mirrors. Generally, most of the prominent websites that does this does not have to comply with any additional regulations. In fact, if a government sanctions them for doing so, then I would think it'll be a red flag for the forum.
Besides, running an onion version of the forum will require some more resources to reconfigure the server and secure same against attacks. I don't think we are committed to all these now.
It wouldn't be as difficult, after all you'll just be running a mirror that serves content over Tor. My impression previously was that they didn't have a good way to rate limit and restrict certain identities. I recall this being mentioned a long time ago, but please CMIIW.
CF does it better over clearnet, albeit being a big PITA for Tor users with captchas. As mentioned, CF can already serve content within Tor (alt-svc mentioned by ABCbits) which is why there isn't usually captchas until the site gets attacked.
If PoW with Tor works, I think having a dedicated onion site without CloudFlare but with PoW would be a possibility.