I had the same problem with a trident maple many years ago. I believe its a fungal problem. Try a foliar antifungal. Maybe a drench too. Hopefully more people with tridents chime in here.
I was at Dave's place a couple weeks ago and he does made custom slabs. I believe he was making one for somebody that looked to be about 24". I have one of his earlier slabs and it is very sturdy.
I Started this air-layer back in April ( 2 months ago) . Today I separated the air layer and placed in training pot. Will make a beautiful tree some day. Wish me luck!