Asking for a free-standing undermount bathtub is akin to ordering sushi "well-done."

Edited to add: I found this photo. Is this what you mean? I think this bathroom may be larger than 10x25 feet.

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Have you (or your wife) found an undermount tub that you find "comfy?" There are way more free-standing tubs than undermount tubs on display in showrooms, simply because they do not require a structure around them to display.

The quartz surround will require a full slab of material, unless you allow for seams at the corners. If you choose a quartz with bold veining or a busy pattern, the seams may be noticeable.

"Modern" and "comfy" are often contradictory. As much as I like the look of Italian modern couches, I find that the seat is too low to the floor to be comfortable. I chose this Devon&Devon free-standing stone-resin tub -- it has a traditional look but can feel at home in a modern setting. (That's a solar tube above the tub, not a giant pot light.)

Yeah, we wrestle with form v function probably moreso than most. But a comfortable bathtub is one of the few things she's digging her heels in. Yes, we have found one, and I'll probably do something similar to the first picture you posted. Not worried about burning a whole slab in the process. At this point, the budget is so far in the rear view window that you can't see it.