As Smitty Two says......
it CAN be done DIY, it just takes some reallocation of time & effort, some planning and learning some new skills.
There is a guy on this newsgroup who proposed digging an additional 3' in his crawlspace to improve working access, I estimated the job in the neighborhood of 50 to 100 days of hand digging. He developed a process (with some craigslist equipment) and eventually got it done. A hard dirty job but accomplished with continuing effort.
Success or failure on this project will be determined mostly by whether or not your wife has total "buy in" or just says she does and then becomes unhappy. :(
My buddy & I have re-done a couple kitchens and in our experience the lady of the house gets grumpy after a certain number of days of inconvenience....depending on the level of inconvenience (no tub is minor, no shower is worse, no toilet is a problem)
I've nearly completely gutted a kitchen & the ajoining laundry room; per my wife's request.
BUT all major functions still work; stove, sink, fridge. I've removed layers of linoleum & plywood down to T&G fir sub floor, modified the plumbing, gas & electricity such that she can get a feel for the new increased floor space & try out (full scale prototyping) different but fully functional layouts. The stove & other pieces are on wheels and can be moved around at will. She's pretty happy with the current fully functional but still gutted layout.
My point is, if you can demo & still maintain function ..... you can stretch the job out. Which is helpful for a novice DIY'r and easier on the pocket book. Just take a some time after every effort and clean up.
Your bathroom space is tight, so you have limited design options. I'm guessing that there was a pedestal sink originally? The stall shower, toilet location and tub don't leave much sink space so I can see your desire to "lose the stall shower".
You a finalized bathroom layout...imo the window in the tub (soon to be tub / shower) area is a bit of a PITA. :( You might consider (from the door) this layout along the plumbing wall; tub, sink, toilet. Relocating the toilet might be a problem but its current central location limits design options.
You might be able to hire an experienced person on an hourly basis to provide help & guidance.
cheers Bob