Howdy neighbor, nice backyard man. I would caution on the side of slanting your seating in the spa There was someone on this forum who designed it this way and said they have issues with floating when they attempted to lay back. I'm sure whatever you decide will be awesome!
That's exactly what I was thinking. Our spa was accidentally built about 6"'s deeper in the center than we expected. Turned out to be a great mistake as we have enjoyed swimming in the spa all winter. If we sit upright, no problem, but if we kind of stretch out in the spa, we notice that you float around a bit with all the air and water flow. If you go after this idea, I'd suggest you consider foot rests to hold your position.
Noise for the spa is equipment related for the most part. We intentionally put equipment 90' from spa and around the corner of house and love it. So quiet and relaxing. The only other consideration for noise is amount of air blown into jets. We have a single powerful blower motor. When it's on, the bubbles are downright noisy (12 jet spa). We will sometimes turn the blower off after it has run for a minute or two. the jets still function as a venturi and push a lesser amount of bubbles. Not as theraputic, but nearly silent.
We love our sunshelf. Recommend some type of lighting on top of it so your whole pool lights up.
Looks like you have a gap between spa spillover and right wall of pool. Might turn out to be a difficult spot for maintenance.
Two LED's in the main pool might be sufficient ... but clearly you have a substantial pool budget. Why hold back
You will need some major horsepower to run all that. " What is the limitation with built-in spas and jets?" No limit, but you need to have the right ratio of flow and air blowers per jet. Some jets are 10 GPM, some are 20-25 GPM. Only other consideration is that your spa plumbing guy will hate you ... and will charge you or your PB accordingly.
"From what I've read, this can be helped by over-sized (or maybe right-sized) plumbing lines." I think this is usually an issue when you are concerned about making sure you have sufficient flow to jets. Bigger pipe = less resistance = more flow. Don't see how this could be related to noise ... maybe someone else can chime in.
BTW, I think that's an incredible design/patio and hope you start tomorrow so we can see it take shape.
Best of Luck
Scot