I play SA based system with 2♦ multi and 2M destructive openers.
I am only trying to say that you should NOT introduce unnatural gadgets into your system BECAUSE the system sometimes brings a disaster, at the cost of natural bidding.
I recall from the beginning of this thread the suggestion that 1♦ and 1NT responses over 1♣ in SAYC would be completely changed to talk about points. This attacks the system philosophy, changes it's natural "basis". When I say "keep it simple and natural", I don't care what your 2♦ opener promises, but I want 1♣-1♦ to show diamonds and I want 1♣-1NT to show a limited balanced hand with no interest in majors. There might be slight variations about what those bids exactly promise/deny, but basically the bidding will be still natural.
IMHO it is not a good idea to merge "strong club and negative diamond" structure into a "natural" (does NOT equal convetion-less) system, that's the whole point of my post.
Having 2♦ as 18-19 balanced will mean that 2NT rebids are 20-21? What does it solve? You'll open 1♣ with 20-21 balanced against nothing and will MAYBE end one trick better off than with 18-19. If your LHO has trapping pass in clubs and your RHO sees it and passes wisely, you're still toasted. True, 20-21 is less likely than 18-19, but, then, the 2♦ may get you higher than where you belong if partner is broke, so I am not sure if it helps more often than it hurts

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