Shove shouldn't push enemies AND knock them prone, I don't think. It should be either or. Either push them away OR knock them prone. Doing both every time means ranged attackers have disadvantage and friendlies might end up out of range for melee attacks.
Cunning action: shove is a good idea in principle, though it does falter a bit on rogues usually not being high strength characters, and at some point we need shove to end up being 5E compatible, meaning an athletics check versus the target's choice of athletics or acrobatics. Being able to place apply poisons and whatnot with a bonus action using sleight of hand rather than attack sounds like fun, but disarming their weapons that way would be OP, I suspect. And it still leaves ranged thieves a bit weak. Maybe a cunning action: Throw?
But I agree that things like triple-dash or having two off-hand attacks will need to be sorted out.
