First of all, you still haven't specified whether your floor is "not flat" or "not level." Whatever else you have (or have not) done to get where you are, it would help a lot if you could say whether your problem is level or flat. Also, just to be sure here, if I am understanding you correctly, you installed the 2x4s ACROSS the 2x6s that make up the underside of the house? (Notice the very very NON-technical terms...so be aware that I'm not a builder or knowledgeable in this stuff like some of these people that post here...just sympathetic to rising note of panic in your posts....) The only reason I'm trying to clarify that is because I am wondering whether you installed the 2x4s "upright" (so the "4" dimension is vertical) or "laying down". I'm not trying to pile on to what you have *already* done, but I don't know what the proper distance for SPANNING that 2x4s have, and I don't know whether they can span two full feet if they are flat. This makes a difference because you want to install tile. If your subfloor is bouncy or bends ("deflects" I think is it), then all the tile you install will just crack and plague you and your heirs until the end of time...until you rip the whole floor and subfloor out again and re-do it. Or put in linoleum. Whichever comes first. You say in the top post that the cracks are like 1/4", but in your last post you write that they are up to 1". That's a big difference? Which is it? Does it vary from 1/4"-1"? Have you actually measure or are just eyeballing or going from memory? IF YOUR CRACKS ARE VERY SMALL: IF it were me (and it isn't), I'd probably just put down a very tight layer of Wedi board (thinset and screws/washers), seal the seams so it can hold the next layer, then put down a thin layer of self-leveling compound (though you still haven't said whether your floor is wavy or slanted), and then tile. IF YOUR CRACKS ARE 3/4'-1": Maybe use a layer of plywood (which would be cheaper but harder to cut), maybe tape and thinset the seams (ask someone on this board about this because I'm just brainstorming here), then use the SLC, then the tile. Only plywood will actually help form a floor (which I think you need to do with 1" gaps), not backerboard of any kind. Instead of Wedi, you could use probably any backer board (ask someone here what to do for the seams). But I only know about Wedi because that is what we went with, and I like it because it is light weight (good if you are a female working alone). I don't think your solution is in one step. It feels like you are going to have to create a tight subfloor, THEN level or flatten. I think you have a non-standard thing going on there, and that might not be addressed so easily with just one step before tile. One more thing, just out of curiousity, WHY do you think your floor must be level? Oh yeah, and I would start making your peace with a threshold. Given how non-standard your set-up is, AND the fact that this is DIY (or at least the subfloor), I'm not sure it is realistic to think you can get away with no threshold.

If you're going to use tiles at the edge to form a water resistant barrier, I see no need to fill in behind them. The tiles you install are unlikely to be as thick as the old baseboard. How will you address the gap between the floor and the new tile base? Perhaps a better choice would be to install PVC baseboard and seal the edges with a sanded caulk to match the tile grout color. Unless you have a drain in the floor, making the room a drip pan will only work until there's a leak big enough to flow out the door.

Are those metallic elements really approved for use in a shower? I anticipate green streaks to follow. 0.o Love the overall look here. If those tiles are really wet-approved, then I guess you have two main choices for the floor. 1) a marble-look porcelain tile with not too much veining. seems like it would harmonize here. 2) simple white or light gray tiles that don't really compete with the other elements here. I'd lean light gray, as I think it would make the white parts feel even whiter, and there are some tiles that are very light "concrete" looks that would have a midcentury feel that would fit perfectly. plus, gray is better upkeep (less discoloration over time) than white, especially the grout. Gonna be a great-looking bath!

IMO the floors should be the same and your inspiration pic has a very simple stacked wall tile with that floor and that is what I would have done in your shower too I think the 2 patterns will compete with eachother and the dark grout on the walls will make it even more so