I've tried strap-less carving swells downwind, but feel more planted and secure in the straps. And I can pull up on the straps to carve tighter

Straps help counteract the sudden foil acceleration on the waves/swells and the uneven sail pressure when turning crosswind to downwind and back.
I think windfoilers are probably going faster than sups or prone foilers as we're going a longer zigzag path to ride the same swell?
I moved the straps closer to the center-line of the board, with the back strap in particular nearly in the middle of the board. Also have the straps very loose.
Coming off is more sketchy re. ankle/feet injuries, but (barring plain bad luck) the risk diminishes with TOW