I'd go with a large-format gray for the floor, which would get less grimy looking and help ground your other finishes. A light gray, not as dark as the pennies.

If you do a niche in the tub, I think I'd try to just use the pennies.

I like a contrasting niche as much or more than the next person, but I think in this case I would use the wall tile in the niche. Let the penny tile over the sink be the star. Make sure you have a very competent tile setter as this forum is full of posts detailing penny tile installations gone wrong.

I would do at least an 8 inch floor tile, in a softened pattern. not a fan of penny tile, takes a pro to avoid the 12 inch shadow line on the sheets. I prefer the same tile for a niche, why draw attention to a utilitarian cubby... unless it is a show room, your shower products don't need to be showcased.

I do not see the floor in the inspo pic but usaully the fwer tile sizes and shapes the better in a bathoorm and I do not like shower niches and for sure not dome in an accent tile if you insiast on one .I aslo think penny tile for a backsplash is not a nice look in fact i dislike penny tile almost alwyas since hundreds of grout lines is IMO never a nic elook. I use 12 x24 porcelain tile in a color you like for both bathroom floor and shower walls then the same tile only smaller for the shower floor. I think you can do anice coordinating tile for the backsplash it is easily changed down the road . The fewer finishes the longer you will like the space and it allows a change in wall color accessories without major work I prefer a shower caddy over the wall in many cases I have encountered the niche has been the source of water damge .I have no idea why shampoo bottles are a decor item we just bring ours in when we have a shower and put them back in the cabinet when not in use

I agree with the above--a large scale with a soft pattern. Maybe this one from Bedrosians?

https://www.bedrosians.com/en/product/detail/floor-tile/urban-2-0-tile/?itemNo=STPURBNOV1224&queryid=3fb9d731cc0ca821470b2bb1873494b8

And I'd do the niche in the same tile as the shower; that's the more updated look.

How about white penny rounds for the floor? Or just plain white 12 x 12's? Either one would work.