Here is how I erase unwanted polygons, to lower poly count in overlapping areas in subtools, make holes in the middle of surfaces, or separate the hairs in a horse’s tail:
Mask the unwanted areas, invert the mask.
In Visibility, click “Hide Pt” to hide all unmasked areas.
In Subtools, click “Delete Hidden” to make them all go away.
Unlike clip masks, this leaves the ends of objects hollow, which may not be what you want. But clip masks have a tendency to leave those flattened extrusions on the model, which can be frustrating.
I wish the Eraser brush worked in Edit mode! Wouldn’t that make life simple?
~Marcia Cash, Traveler in Thyme