I am getting ready to grout a walk in shower so I have had to deal with the same issues as you. I spent a lot of time worrying and thinking about the transition. I will give you something else to worry about. In the previous bathroom I used 3/8 plywood under ditra. Proper spacing on screws etc and 2 days after applying the Ditra I noticed buckling in the surface. The plywood buckled in places between the rows of screws. I will never use 3/8 in again.
You are going to sister the existing floor joists and put plywood between the existing floor joists. Nothing says they have to be flush with the existing ones. What if the were a bit higher and you put shims on top of the existing joists to fill in the gap. I did not think of this until it was too late. When I put kerdi band on the joint between the tray and Ditra I put some extra mortar on the tray right at the seam which smoothed out the transition some. I then put on a extra band on kerdi on the tray right at the seam to build it up a bit. You also have to account for the differences in tile between the bathroom floor and shower floor. A 2x2 tile is going to have less mortar under it than a 12x24 tile.
I am still looking for the Schluter video on how use their shower trays when everything does not match up exactly