Hunter PGV diaphragms tearing

The pre-work you've done lays the foundation for a great system. Good job.

Use a pressure regulator

The length of throw, the precipitation rate, and the GPM flow requirement quoted for a particular sprinkler all are based on an assumed pressure. That's commonly around 45 PSI, but check data or information sheets from the manufacturer to be sure. If you can't find published data then move up to a higher tier of sprinklers or change manufacturers. When a sprinkler is run over its nominal pressure it'll throw too far, make more noise, atomize the water smaller (and thus be more susceptible to wind drift), demand too many GPM..

Winterization (Drain or not?)

There are two schools of thought for winterization: design the system to be drained or plan to blow the water out with compressed air. Draining could be manual or automatic. Either way draining takes up-front planning and somewhat careful construction. Automatic draining is maintenance free (unless/until the drain plugs clog). It does consume more water because all the pipes are filled and drained with every operating cycle, and it's more noisy (popping and hissing) when the sprinklers first come on and expel the air from the pipes. Manual draining and blow-out both avoid those drawbacks but require attention every fall. Blow-out can require a sizeable compressor -- a pancake style won't do. If you choose blow-out or manual draining then you don't need the automatic drain plugs etc.

Burial depth and slope

When rigid riser pipes were the norm burial depth had to be kept within about 6-10 inches. With the connection to each sprinkler made by swing pipe, often called by the Toro trademark "funny pipe," burial depth is largely a matter of preference or convenience.

Slope really only matters in a manual-drain system. It matters little if winterization will be done with compressed air. In an automatic-drain system you can figure out where the low points ended up and add automatic drain plugs at those places. Remember that those plugs eventually fail, though, so document their locations -- but consider controlling the slope of the pipes so that the auto drain plugs can be put in locations that you prefer. Maybe even put them in a sprinkler valve box so they're easy to find and access for inspection or replacement.

Other thoughts

Overlapping coverage

Overlapped coverage is a good thing. Do you notice that the drawing shows two quarter-circle heads overlapping to cover the top-right quadrant and another two quarter-circle heads covering the lower-left quadrant? That's good. Now look at the upper-left quadrant. There's just one head covering much of that space. To keep the grass green there, ie to get enough water in that quadrant, you'd have to water until the other two quadrants become marshy.

Fix it by adding more heads. Add a half-circle head in the middle along the upper edge and another along the left edge. Finally, change that one weird little sector at the interior corner by the house to a 3/4 circle.

Choose a sprinkler model that has a throw radius about half the width of the yard. Distance can be fine-tuned for the whole system first by adjusting the pressure regulator and then by adjusting a screw on each sprinkler.

Precipitation rate

Precipitation rate is the amount of water (the depth) delivered in a period of time. It is measured just like rain in inches or millimeters or whatever per hour. Some sprinkler families are designed to deliver matched precipitation rates, and it's worthwhile to choose one that does. Often the rate is set by changing out a nozzle to control the GPM. You could use a 1 GPM nozzle in the quarter-circle heads, 2 GPM nozzle in the half-circle heads, and 3 GPM nozzle in the 3/4-circle heads (for example).

Zoning

Avoid the temptation to put too much GPM demand on a zone. If you add heads and choose the flow rates as I suggest the total demand in the turf grass would be 5*1 (1/4 heads) + 2*2 (half heads) + 1*3 (3/4 head) = 12 GPM. Divide that into three zones of 4 GPM each to avoid straining your water supply, which manifests as sprinklers not spraying as far nor as well as they should.

Water needs for shrubbery, potted plants, etc are different than for turf grass. You'll probably do well to put all of those together on a zone that does only these and no grass.

It looks like you have some "wiggle room" - the existing valves could be pushed closer together, though it might require rebuilding the manifold (branches off the supply line).

My 7-zone system is indeed in a single rectangular box. For ease of identification, I would recommend (at least) labeling each zone valve and if at all possible, re-number from 1 to 8 consecutively to avoid future confusion.

For your zone 8 to point the other direction, you should either route it around after it exits the box or dig deeper and route it underneath the valves. The valve should be in-line with the rest of them. Use care not to exceed the maximum bend radius for your tubing and do not kink it.