Have you considered redesigning the main pool so that it better suits your needs? You can make it smaller or shallower or both. Consider hiring a landscape architect to redesign the backyard space.

Seems like a bit of a crazy idea! Just build a hot tub with jets. People like alternating between cool pool and hot spa.

Do you know where all the pipes are for the pool? Depending where the pool equipment is, there might be lots of pipe under that grass area.

It also looks like you have a retaining wall around the back side, so doing more excavation might not be so easy. Messing with retaining walls might also trigger any hillside building rules you might have in the area, so check your local zoning/permit requirements.

A hot tub is what you need and IMO that space looks too smal for any real hot tub even since there are , at least where I live, rules abot spaces around pools . IN fact I like hot tubs close to the house too. At least post the size of that spot it does not look very large . I aslo totally agree you need professional help with anything going there . Not even sure how you would excavate there without some issues

A solar recycling water heater on the roof will heat the pool on nice days. All that is powered is the water pump to pump the water up to the roof and gravity brings it back down to the pool.

The sun heats up the water. My BIL can get their pool well over 80 degrees with this system. 80 is ccmmon and very swimmable.