Upside Down triangle would look like yet another unambiguous shape I believe.
On my hand-drawn (so glad its over) diagrams I'd denote Maj7 step as delta (your existing triangle) and dominant 7 as upside down triangle... Regardless, by adding effectively 2 more shapes, we end up with 10 shapes all in all - enough to cover practically all possible alterations visually.
Not sure if anyone requested custom shapes - just wondering.
Thanks,
Sasha