Looking at the construction of this floor I'd say it's very well put together, right down to the quality of timber used in its construction. Bearing that in mind I doubt that there's gonna be much deflection at all in the floor. That said the only movement that would be worth worrying about is going to be lateral. I wouldn't rule out build up to rsj height with appropriate thickness of ply, then a complete over board and then again with Ditra stuck with AF200 on top of that. That would mean that one exspainsion joint in the centre of the room splitting it into two 7.5 x 7.5 bays to accomadate the tile plain in itself would suffice. It would look a lot better than four exspainsion joints imo and worth the additional cost of the Ditra. To cover themselves Schluter etc would tell you to have exspainsion joints all over the place and nobody can technically argue with that advice for sure, but having successfully done the above myself a few times in my experience it would work fine.
