I’d say, or rather, what I’d do, is just get what I want to play now for the system I own, and then if it’s awesome enough just get it again on the other system. Sure, that means I have BotW for example twice but whatever.
So; I’d get Horizon now on PS4Pro, then if I thought it was good enough I’d get it a couple years or whenever down the line for PS5 as well when I’d have that system as well.