In the end, i'm not sure limiting short rest is really necessary... The RAW hit dice system create a limit by itself.
It doesn't require additionnal rules / mechanical limitations imo.
If we don't regain HP when we short rest because we're totally out of hit dices, players will have to long rest soon. So I voted 6 but for "something similar" in mind.
Hit dices sounds the most fun ...
ON the other hand they add yet another mechanic to watch. :-/
"It sounds funnier but I don't want more fun if it require any effort"
