Labor costs depend on your location and materials cost depends on the brands you choose. I'm replacing a tub, toilet. tile floor & surround, vanity & top & all fixtures. Not moving any plumbing or removing any drywall. Using mostly Kohler products. In Massachusetts it will be about $20,000.

Agreed, the cost will depend on your location, the amount of work you have done (moving plumbing or electrical will be $$$ more), the quality of contractor/workpeople you select, and the level of materials you select.

I'm in North Carolina and in the very rough planning stages of an 8x5 (40 sq ft) bathroom. I'm budgeting $25-30k.

If I was going to go with big box store medium level finishes I'd budget $20k.

The most important aspect is that you find the best, most qualified contractor/workpeople. Just look at Houzz discussion posts to see all of the horror stories that happen when buyers go for the cheap, inexperienced workpeople. Or "experience" in doing everything wrong. You DO NOT want someone building out your shower who doesn't follow proper waterproofing.

Bathroom shown is quite compact, what you have now is about the best to be expected (in the space you have) unless you are willing to; eliminate the tub and the window, and move the WC.

I’d put the shower where the bathtub is. And if you must have a tub, will a freestanding fit where the original shower is?

As others have said, your location and the COMPLETE scope of work is required. You will obviously tear out everything, including tile floor, shower receptacle to the studs, vanity replaced, lighting changed, etc.... Research carefully for the qualified contractors and get estimates, and be prepared to wait for qualified labor (and plan on longer lead times and higher prices than several years ago for fixtures and materials as well). Plan ahead and spend your money wisely so you are not re-doing substandard work in the future. What's your budget? Start there to determine if and what you can do or if waiting to get what you want and a quality outcome is a better approach.