OK, I can't see going beyond a 17P SS 3 blade. Double cup, wide(er)
blades, maybe down 1/2" in diameter at most.

If you can get to a popular anchoring spot in season & strike up
conversations on props, you may find some folks who would like to try
your prop & your spare, and you theirs. I'm assuming your an
Alpha(drive), so there will plenty of those around.

Good idea.

What's the bennefit of going down in diameter?