Modified mortars are more resistant to cracking? Well, that's not true all by itself. It's true that some modified mortars are more flexible. And if there's some differential movement between the substrate and tile, it may flex enough to keep a crack from being inflicted on a tile. But you've installed your tiles per Schluter specs over Ditra. If some differential movement occurs, the Ditra is there to relieve the stress between the two. You did good.
As long as the 2x6 joists are up to the L/360 deflection rating, you're good to go. Take a look at this
Deflecto calculator to see how your floor structure rates.