24" cabinet for sure -- I like the 24x30 size for actual people storing actual stuff. if you're using a light that low-profile, be sure that your cabinets are recessed; otherwise, get something that will stretch past the top of the cabinet to light your face.

I remodeled my bathroom last year and installed oval sinks. I don't care if the rectangular ones are "in," I don't like them either. In five years they won't be in anymore anyway. I think oval sinks are still being done, it's just that some people use the rectangular ones. I also have an oval tub and I have a transitional gray and white bathroom as well.

First question is always "what's your budget"? Other than the stained floor, nothing appears really wrong that styling won't help. So you may be able to achieve an attractive space without spending the world. See if you can have that terrazzo professionally cleaned and sealed. If that's possible, it's likely the easiest solution and will save a lot over the cost of replacement. The floor tile and vanity are cool neutrals. Replacing them doesn't make much sense unless they are broken. But the paint in there is a warm neutral and clashes terribly. Consider a pale blue or light grey. Consider adding more lighting. Updating the towel bars and existing lighting fixture/fan would help too. Then buy some towels that are a dark version of whatever paint colour you chose. That will add a bit of necessary contrast

Saunas, especially dry ones with far infrared heat, are touted as great for health and for sweating out the impurities that we ingest. Others say that they are good for arthritis pain. If you want to fit one into your home, I say, go for it. Get yourself a self-contained one, cabin style. Put it in your bathroom if you want to, or put it in the basement or in a little-used guest room. I have mine in our smalest bedroom, which we now call the Sauna Room. I have not used it in four years - since the bath romodel next door had me stuffing it full of stuff that needed to be tucked out of the way during renovations. (We also widened all our doors to 36” during that remodel, so every bedroom had some construction in it.) I am not sure I ever really noticed much improvement when I did use it, but I neveer did use it regularly. If you were wanting a more traditional indoor sauna with the bin of rocks and wooden pail of water and wooden water ladel, you might want it in the bathroom to be close to water and for the fan to deal with the steam. But if you are not doing that type, you can put it anywhere. If you go to sell, put it away, or leave it there and show how a small tub could replace it, if you have it in the bathroom. You could even do the plumbing for a tub and leave it in the walls, photograph it with a tape rule, showing its position in the wall so that you can tell buyers the plumbing is already in place. Bring a drain to the general area and measure out exactly where that sits under the flooring, too. Have a box of extra floor tiles to leave buyers if they have to open the floor. When you do weird things to your house, you can have the ”traditional” stuff under the surface, waiting for a new owner to uncover and use it. I did this in our main bath when I got rid of the second sink. The plumbing is in the wall for it, and I have the photos!