> It's pretty wild to me to imagine that for all of our diversity and abundance today, there was much greater diversity of homonins at a time when we were much fewer in number.

Fewer in numbers means more isolation between groups.

In addition to that, early stages of culture may or may not have further augmented isolation. This is purely speculative of course, but we are in "imagine" territory anyways. Then the first group to develop cultural traits that somehow facilitated crossbreeding might have triggered a cascading melting pot effect, leading to the relatively uniform "modern human".