As much as i'm gonna get hate from the smaller sub creators for this, i'd put a subcount minimum or a like minimum and a page minimum to make the cut smaller. You'd want a contest with comics that have more than two pages and people that actually read it. That's what i would do.
(Even though i might exclude myself with this not that i would win this anyway)
You could do a genre limiter but i personally think that is a bad idea as you mentioned because it takes away the diversity of the contest.
Also i agree with @faeriesandents to use the google forms for this. It prevents certain users to act like "YOLO a free reward lets go dump this link here!" and walk away.
Do you have in mind about how many people you like to participate?