Replace the window with a vinyl window.

Make sure the rough-in stool is sloped toward the tub 3-4 degrees is enough.

Wrap Kerdi backer cloth into the window opening to seal any possible leaks.

Tile up to the window over the Kerdi cloth..

Your contractor should know the detail of how this is done. We do it all the time.

What are you going to do with the floor register?

Why move the tuba at all? It looks nicely at home where it is.

Moving it would be more work and expense than it is worth.

The glass must be tempered safety glass so whether or not you replace the window, you must replace the glass

Thank you for the response! Agree probably not worth it. Trying to find best layout in this small bathroom. We do have an extra closet we can incorporate for extra space, if needed.

That would be great if you could add space, depending on where the space is located.

It appears that the current layout may not meet code, although no measurements were provided. The minimum clearance in front of a toilet, tub, or vanity is 21 inches. Renovating the bathroom will require bringing it up to code.

I have a shower/bathtub in front of window (1945 house) .I used a film on the lower half of window (double hung) for privacy.