[Zeke]

I'll second going down to bare joists. You can do your slope on the joists. After making sure you got all the nails out, a power planer makes short work of sloping the tops of the joists using a laser as your guide. Better check and see what slope they want. Standard is 1/4" in 12 inches. But with ADA in mind they might have their own spec.

What I have done in the past (I think I took this plan from Trekkor) was lay 3/4 T&G flooring on the joists, glued and fastened with your favorite fastener. Then after pre-cutting all the cement board and laying it out, I used thinset spread with a 1/4" notched trowel but not square notches, don't need that much, and laid the CB into the thinset and rolled it out. I waited on fastening the CB until the TS was hard to eliminate any waviness. Then I countersunk CB screws. This is NOT a wet floor. But the CB over the subfloor set in TS does not flex. Solid as a rock.

I went down to the joists for height reasons. I was adding 1-1/4" to the existing joists which matched up to the original hardwood flooring in the house floor is close enough (3/4" plus 3/8ths wood). Add in some for the thinset and I got a close enough transition that it isn't noticeable . I may have shaved a bit on the joists to get a match, it was 2016 when I installed that floor, so memory fails.

AFA sloping design, direction and waterproofing, that's a job in itself. You likely want to avoid the obvious path in the room to use the facilities. I believe you need a 2" line and that's 2 plumbing units added to your sewer use. It will be interesting to see where you come out when the floor sinks are installed.

I know the aesthetics are discarded for this thread, but I have done low fired tiles in commercial and it was a disaster. High traffic areas were compromised within a year. I used porcelain tiles in '16. That or quarry tile for the deep red color of terracotta, not to mention again the transition problem with thick terracotta. I've got no use for the stuff. [/rant]