1) You can brace or block the joists per standard construction, but what might be easier is to strap the bottom edges of the joists. Typical strapping is done with a 1x3, but a lot of times I'll simply use a 2x4. Run a 2x4 across the bottoms of the joists, perpendicular to the joists. Add a dab of PL adhesive and use a couple of screws at each fastening point. You have specific needs here, so spacing the 2x4s as you need will work very well. What it does is if a single joist wants to deflect downward, the strapping transfers that load to the adjacent joists as well. So it's good as a floor stiffener. For a washer/dryer set up, it also dampens the entire floor platform in terms of the possibility of the spin cycle causing the floor to reverberate. The strapping serves the same purpose as cross-bracing or solid blocking between the joists. It's code-approved, though code only requires it for 2x12 joists, with the strapping being made of 1x3, and the strapping spaced every 8'. The strapping is more effective long-term than bracing or blocking, due to fitment, installation, and shrinkage issues. 2) Do the whole floor. There are only restrictions on maximum spacing (every 8'). So if you make it every 2', or 3', or 4', no worries. Strapping is also easier because you're not wrestling with any of the utilities that take up space between the joists. 3) Your joists span 8-1/2'. For simplicity, why not do run three pieces of strapping? One at 1/4 span, one at half-span, the other at 3/4 span. Just a little over 2' spacing. 4) 3/4" ply over the diagonal boards is plenty. It does not have to be exterior grade. 5) Your floor sandwich can be made up of: -Tile -Ditra -Floor heat wires encased in SLC (no wires under the washer/dryer area through, see comments later) -Plywood underlayment covered with SLC primer -Diagonal plank subfloor -joists I usually advocate a small platform under the washer/dryer stack. It's more for damping than load spreading. I've done sandwiches of plywood/cement_board/plywood, or plywood/MLV/plywood. MLV is mass-loaded vinyl. Screw through the top layer of ply, through the middle material, and into the bottom layer of plywood to secure it together. Or just use construction adhesive. I drill a hole through the middle of the platform sandwich to accommodate the flood pan's drain line. I usually put a square of rug pad on the floor. The inexpensive, maybe 1/8" thick, rubbery mesh or open weave type of rug pad. It acts as a bit of a damper, but also holds the platform in place. I then set the platform on the rug pad. Then put the flood pan over the platform with the drain plumbed through the hole in the platform. Then set the washer in place and the dryer on top of that. I recommend you tile the entire floor, but again, no heating wires under the location of the washer. You'll be putting the sandwich platform on top of the tile in that area, and the insulative value of that platform can shorten the life span of the heating element. Pricklypear, the Ditra-XL is simply thicker than regular Ditra. It really wasn't designed for a specific tile size. It's sole purpose is to allow the thickness of "Ditra plus tile" to match up with the thickness of a typical hardwood floor.

This post has some misleading comments regarding ecocork. I am investigating underlayment for vinyl plank. If you look at the actual test results from NGC testing services for the delta IIC of quietwalk LV, floormuffler lvt encore, and ecocork, they are all tested using the same 6 inch concrete slab (not 8 inches). The IIC and STC have other included materials in the test like insulation, gypsum ceiling, etc. But the delta IIC has only the concrete slab and measures the difference in sound dampening with and without the concrete slab in place telling you the true dampening of the underlayment itself. By this measure the quietwalk comes out on top with a delta IIC of 28, ecocork at 23 and floormuffler at 22. If you look at the frequencies, you can see that the quietwalk is the only material that has some degree of dampening of the lowest 100 hz tested around 5 db. The testing is slightly different in that the quietwalk was tested with a 5 mm vinyl plank on top whereas the ecocork was tested with a layer of engineered wood on top. The thickness of the engineered wood was not given. Floormuffler does not have a link for the actual test results. This article has a helpful explanation: https://arcacoustics.com/2016/08/23/stc-iic-delta-iicwhats-this-all-about/

@Lisa G If you spoke to Mapei im sure they told you self leveler over a wood structure is risky. especially when using it to level. There are very strict minumum or "feather edge" concers with self leveler and if by saying " mesh " you are likely referring to " 2.5 Lath (synthetic or galvanized) its very tough to terminate that with a feather edge. It usually requires a deeper pour or Lift to encapsulate the lath and meet mapei's mimimum thickness requirements " While terminating leveler anywhere in the field of the floor. ( or perhaps they are referring to bridging board seams with sheet mesh as well..) mapei does make a few different levelers as i recall. Regardless. You have a bigger issue in either busted joists, crowned cantileavers, or a sil plate, support beam issue etc. and that needs to be dealt with properly FIRST. simply jacking the floor up at low spots im affraid is not always the fix. At times we have to completetly remove all subflloring down to the joist themselves and plane down joists, cantieavers etc while also sistering and raising other settled joists. This can also involve failed footings, CMU issues rotted sil plates etc. ( i am not looking at youre homes structure here). Everything Cancork said about Mapei and OSB OSB OSB is very true and remember that self leveler used over a deflective substrate still requires a properly installed and deflection rated substrate to work and it does not like feather egdging at all over wood as it is a great point of possible cracking/bonding/failure. I would suggest getting a lot of stuff in writing and document things with a lot of pictures of both under the home @ post/beams/joists & cantileavers and sil plates/foundation/cmu. In wood structured homes the best subfloor/substrate you can apply is EXP1 APA exterior glue plywood, double layered at 1/2 or 5/8" layers cross grain, glued and screwed and this is likely what mapei will tell you. For floating and wood floors you can at times get by with a single 3/4" layer (structure permitting) as well you need to be sure all and every board seam being properly perimiter and cross blocked. Self leveler is great in the right scenario, but remember its not being used to add strength ,(the lumber and plywoobut are) but the leveler is only there to level out an ALREADY sound and properly prepared subfloor/substrate and it is ADDING more WEIGHT to youre joist system and substrates deflection.. sorry for the long winded typos, a subject near and dear to every flooring guy/gal!!! My free advice learned over years THE HARD way, the kick in the shins hard way!!. The thing about what we do is we learn immensly by failures, and tearing apart homes to see exactly what works over time and what doesnt. as well as staying on top of constantly changing industry standards and practices. Cancork included. Perhaps you have an excellent builder, still tread lightly with levelers over lumber and try to find the structural cause of the out of level floor.- best

Hydrocork IS a click together floor! So whatever the subfloor issue is, it needs to be fixed. A "long yard stick" like piece of wood is only about 3-4 feet. That is A LOT of height variation over 3-4 ft (1/4" - 1/2"). I'm sorry to say but you need to remove the hydrocork, have the subfloor dealt with PROPERLY (laying a new layer is your best option) and then starting again. The original installation contract SHOULD HAVE dealt with the subfloor variations. Almost all subfloors have issues. And the one's covered in sheet goods (like sheet vinyl or carpet) have MASSIVE issues (I'm going to guess that was what was in your space originally, am I right?). And just for fun and giggles, Hydrocork is having issues in the N. American market. It seems the education level of the installers is NOT being dealt with. Hydrocork needs plenty of explanation and know-how. That means a hands-on course. And if the dealers are NOT sending their installers to the courses, then the installers are left in the dark as to how to do this. In Europe there are NO PROBLEMS with hydrocork....because all the licensed dealers are FORCED to send their installers to the workshops. The installers come back with plenty of knowledge. This system doesn't exist in N. America (or very rarely shows up). So the average N. American installer is TOTALLY in the dark about hydrocork. And it shows. Get your subfloor fixed or else you lose ALL warranty. And to be clear, only carpet or sheet lino/vinyl can be installed over subfloors like yours. Nothing else.