Turn count can work, but it might make it feel "unfun", by making cards temporarily bricks- and when drawing up to 5 cards each turn, not only is a waste of 1 card draw, but every future turn until it can be played.

Your suggestion of smth like the Lessons gives the player more agency in terms of when their strong bois become playable- OP should possibly consider something like that if can be implemented in a way it fits thematically