Mind of tempest
(he/him)advocate for 5e psionics
you also have foot-based nomads as were found over Africa, Australia and north America pre reintroduction of horses.I think you can hack it. First, I'd give choices. For instance, say that all human cultures automatically have Prodigy or Skilled as a choice, and then each sub-culture grants one or two other options--like Mounted Combatant or Mobile for your Mongol example. Or make up new feats for each culture. As long as there's a choice, you won't have the problem with someone getting stuck with a useless feat.
then you get the wagon baed ones who I forget the names of.