Seems 'broken' with Envoy, if broken is even possible. It's a bargain bin Throne Room, which is pretty awful.
Envoy is a trick to get around the limitation, but it's certainly not broken with it; it just makes it slightly cheaper to buy Throne Room when you want to use it with Envoy.
I think if you wanted to do some sort of limited Throne Room, then the natural limitation would be based on card cost.
"Choose an Action card in your hand costing less than $5. Play it twice."
But again, it just seems weak. Maybe it could cost $2, but even then, a nerfed Throne Room isn't that exciting.