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.

We have wood in our master and you can tell its been through some stuff. Basically it only shows in frequently water-logged areas, like right outside the shower, and next to the bath where years of wet hands reaching out to the faucet have dripped. I think most of it could have been avoided with better planning, like a shower door instead of curtain, and I think if the previous owners had given it a sand and a good refinish right when the damage started to show it could have made a difference. Even so, I'm surprised how well it has held up, and I love having the warm feel of wood in the bathroom over cold hard tile.

What kind of finish are y’all who are worried about damaging your HWs using? Granted, I don’t have mine in any bath except the powder room, and mine are site finished white oak, not engineered....but in my kitchen my dog often drips copious amounts of water from his fluffy face after he drinks, and my knuckle head teenagers will drop ice cubes and perhaps are not super diligent about picking them up, etc. These types of puddles have occasionally been there quite a while....but I’ve never had a bit of damage. I could totally see having HWs in my master bath. Now, in my kids’ bathroom? No. They have tub/shower combos and are not super careful about adjusting the shower curtain properly. But in general...I have never felt like my hardwood floors are a fragile type of flooring.

Hi, I think if you are trying to limit the budget, painting the wainscot is a great option. I can't tell from the pictures what finish is on the wood walls, but if you choose to paint I would recommend speaking to a paint store about a deglosser or gripper primer, if there is a finish over the wood. BTW, with a new vanity, some paint, new mirror and a light, this room could look fantastic for a bsmt bath Good luck.