When you say crossbeams, is this blocking or actual beams? IOW, what is the maximum length of any unsupported joist? You need the deflection ratings to be L/360 or better, and without the length, there's no way to tell.
You can't tile directly to planks, you need a layer of at least 1/2" exterior glue rated ply (with C or better sides; i.e., no 'D' side). Then, you can use either a decoupling membrane or cbu. What are the dimmensions of the room? With lots of windows, you may need to make special provisions for expansion (about every 8' or so in full sun). Ditra is easier to install than cbu, and after you add up the screws and tape, not that bad in price difference as well. And, it works better, lots lighter to carry home, and cleaner and neater to cut. Ditra XL was designed for really troubled installs, it can work on 24" spacing of joists for example, and the extra height makes it easier to match up heights with typical tile to wood surfaces. The original Ditra would work fine here, since you aren't trying to match surface heights and don't need the extra protection it provides.
No adhesives needed in that sandwich. First, make sure the planks are well screwed down to the joists. Screw the ply down to the planks, avoiding the joists (ideally, have the end 1/4-span or about 4" past the joist for your 16" OC spacing). Then, read the installation instruction manual on Ditra about the proper thinsets to use. Typically, it's a modified thinset to anchor it to the plywood, and an unmodified one on top to set the tile. Keep a gap around the edges - don't butt the tile up tight against a wall. When you grout it, don't grout that gap, either. This allows a little expansion, otherwise, you may have a tenting failure where you literally sheer off some tile as it tries to tent to accommodate for the expansion. If it is wide enough, you need intermediate expansion joints too, the spacing of those depends on how much sun it gets. Ditraset is an unmodified thinset spec'ed by Schluter that works quite well (made by Bostic if I remember correctly). Don't use the least expensive unmodified, as they often barely meet the spec, you want a little more leaway.