The square will always tend to mark because the bit is stationary in X, Y whilst is moves up and down in Z, you’ll also get some deflection when it plunges down which may also take a bite out of the wall.
Taller triangulars may help, depends on the material which way to go.
I always try to leave the contour cuts that need the tabs until last as they really weaken the workpiece, even if things don’t break they can vibrate and give a nasty finish. Where I have a lot of contour to cut I run the toolpaths down to the last 3-5mm of the Z depth first by setting the bottom height to do the bulk of the cutting and them come back in a second pass of the contour going full depth with the tabs.
EDIT - those numbers are for wood not metal