Hi !
I guess the more the domain can be easily remembered, the better. And the shorter, the better. It’s like a brand name, and it’s great if it sounds good, is attractive, and so on. The problem is to find something available. Dot com are probably the most difficult in term of availability, but if you manage to find a good one, it’s still the first choice IMHO.
Also look at the price. I don’t know about “.community” but it may be more expensive. Some specialized TLDs aren’t very cheap (I don’t know for this one).
I personally think the sub-domain thing is a great idea ! You should be able to do this pretty easily with a redirect. You just have to redirect category-name.domain.xxx to domain.xxx/c/category-name