I'm at 15 right now. And I don't see anything as being "cheese" for an optimization challenge like this, anything that the game allows is fair for me. Like, I have a proper solution to behead, but there's no reason why I'd pick behead as a fundamental word (as you did) over between when between exists.
Simplest Box I know is ((?)) format. (edit: wrong word lmao) (well, for the golf metric it's something else with fewer chars, but you're not going to define split without some sort of box already defined.)
Actually, Tac is just ?? format. I suppose that one can be used in place of cat as a fundamental.