noice
Mate with the archbishop in Gothic Chess

There is an interesting twist to this position.
If white tries 1. b3? right away to block the Archbishop threat, 1...Ae7 wins for black easily.
Yet if white plays 1. Kb1 and waits for the punches to come in, if black tries for a knockout with 1...Qa5 then 2. b3 is fine. Scary, but fine. Now 2... Ae7 forces white to find the courage to walk into 3. Ad4 Aa3+ 4. Ka1 Ab2+ 5. Kb1 and black can't finish him off.
I like the line you suggest after 21 Kb1 Qa5 22 b3 Ae7 23 Ad4! Black's attack runs out of steam, and his king appears to be somewhat vulnerable.
Why is 21 Qxd8?? a blunder? What should White play instead?