Zero-K matchmaking does expand its acceptable WHR range relatively quickly, 2 minutes is already a relatively wide band, and it should reach its maximum allowed band around 4 or 5 minutes. Players at both ends of the ladder are a little more likely to get games too.
It is possible to allow users to customize their acceptable WHR range, but this is a little tricky to communicate to users and bears the risk of reducing the total number of matchmaking games if everybody selects a narrower range than the current default, gives up searching, and queues less into matchmaking. To alleviate this, there could be a checkbox to the effect of "give me a game, any game" that skips WHR checks, but this still requires both players to agree, and may still diminish the average quality of games if the matchmaker doesn't get enough time to wait for better candidates to enter the queue.
The pro 1v1 room was added to make it easier for the top end of the ladder to match each other regardless of WHR balance, this approach has its own drawbacks of course and doesn't apply to the rest of the ladder. There are probably other improvements that could be done to matchmaking, but it's a tricky problem given the low matchmaking population in Zero-K.