My thinking is that there will be commercial contexts where an AGPL license is a non-starter --- it's incompatible with the business model. If so, I think it makes sense for them to buy a commercial license from me, even if it lacks features in your fork. If they can't use AGPL code, your fork may as well not exist, except as this tantalizing something-I-can-never-have that makes the main library look worse.

I'd also note that you gain absolutely nothing from maintaining your separate fork, other than the principle of the thing. I'm compelled to distribute the code under the AGPL, just as you are. If your features are compelling you could instead negotiate with me for a cut of the license fee.