Your sketch is a bit cryptic. You may want to show it to a builder to get their opinion on your sketch. If you are looking for design ideas contact a local architect that can look at the the big picture and perhaps come up with a better solution.

Have you looked at a 48" bathtub???

You may get better feedback if you post a sketch of the entire floor with dimensions. Your sketch is really hard to understand.

As-is Room. No plumbing at all, so anything is game. Must have bathtub.

Anyone designing and/or building out the space will want to know a lot more information; such as what is below the space? Where are the closest water supplies? Where is the closest drain? What is adjacent this space? what is the height of the window? What is the view out the window? What are in the walls? What is above this space? Why you want to make this change? What is your budget? Why must you have a tub? And more?

I am not looking for answers to these questions, I am trying to illustrate your need to have someone come out to your house to collect all the information needed to come up with a proper solution.

This is a 5'8" man in a 53" tub. Imagine it in a 48" tub! By the way - I'm loving this sight. How funny! https://www.bathsofdistinction.com/bathtub-size-chart-s/143.htm

what is the height of the window from the floor? Can it be made smaller?

I’m really just in need of layout ideas.

I would rather have the cabinets, but I thought that having the sink sectioned off would be a nice level of consideration for 3 bedrooms sharing a bath. Maybe that’s an old fashioned design concept?

We’re (VERY experienced) doing the work ourselves with a $10-15k budget.

I’m REALLY hoping that at 48” alcove soaking tub will work! This will be the only rub in the house. The adult men of the home will have to take showers if not (master shower is a double.)

And window is front of brick house and cannot be changed. It’s about 30” from floor.