Actually, there are! You can click below those buttons, where it says the name of the "brush," and it will bring up a full on-screen keyboard (looks like it was meant to be used on phones?) and you can type the name of the "brush" you want. There are some cool ones which seem less common than others? I found "Quicksand" and "Recurse", the latter of which I only saw once naturally, but I could deliberately re-invoke it and got a pretty neat effect in my opinion!