Welcome, Thomas.
First, you've actually got as much "wiggle room" as you need there. One of the easiest transitions to be made is from ceramic tile to hardwood flooring with a simple custom piece to match tile height on one side and hardwood on the other.
As always, I recommend you make your subfloor suitable for the tile installation, then make your transitions to other flooring as needed.
I think you might be misreading the Schluter recommendations for installation over sawn board subflooring (which they call structural planks). The minimum requirement for Schluter as for the ANSI standards is a minimum of nominal 1/2" plywood over such board subflooring.
You've given us your joist spacing and a possible range of sizes, but no unsupported span. I'd recommend you evaluate the actual structure to determine whether it is suitable for a ceramic tile installation. You can use the Deflectometer in the dark blue bar at the top of the page to get an initial go/no-go reading. Doesn't matter that you had tile before, which was incorrectly installed, by the way. Doesn't mean it couldn't work, just that it didn't follow product manufacturer's recommendations.
Bottom line: You must install a second subfloor layer of a minimum of nominal 1/2" exterior glue plywood with no face of grade lower than C, then the tile installation substrate of your choice. Your thickness guess for the Ditra installation is close enough. You'll not know the actual final height of your finished floor until you have a finished floor using your materials, your tools, and your technique.
There is no need to level the floor unless you just want it level, your tiles don't care. But your tiles do care that the floor is very flat, and the larger the tiles, the more they care.
Then you can make a wood transition strip to mate the tile to your existing hardwood flooring. You'll want the strip to be as wide as possible to avoid a steep slope and remain withing industry standards and probably your local building code. We don't know where "local" is as you've not added a geographic location to your User Profile.
My opinion; worth price charged.