Trying to have a subclass allow choice of specific spells to augment.

I'm trying to create a subclass that, based on level, will allow the player to select a certain number of spells to give a bonus to. I'm trying to have the player make 2 choices at level 3, then gain an additional selection at levels 7, 13, and 19. Every way I've tried to do this has either allowed the player to make every choice at level 3, or the choices are level restricted but don't allow the choice made to be a spell. Additionally, I'd like to know if it was possible to restrict this choice to spells you already have access to.