First, I have had phosphates as low as .00 and as high as .55 ppm. I've had nitrates anywhere from 0 to 70. My stats right now, trying to get phosphates up. My SPS are doing phenominal. I have a chalice that I have to break chunks off and throw away. I gave a 10" Piece of Chalice away to the LFS and I've still got a 10" piece that grew an inch in the last 2-3 weeks. I several species of acros that have grown into large 8" branching colonies. A massive Hydnophoria that keeps getting thicker and larger. Bonzai Acros that keep getting larger. Montipora continuously taking over realestate.
My alk I try to keep between 8 and 9 (seems to suit my LPS and SPS the most). Calcium is between 500- 525, magnesium is 1500 or so. Salinity between 1.025 - 1.026.
The only difference is I have a 340 gallon vs. 75.
That said, I went heavy into my lights. Well over $2500 in lights alone. I run 3 AI Hydra 52 LED lights in an Aquatic life fixture with 4 T5 High Out put (80 Watt bulbs each). I run the LEDs near at max for the blue bulbs.
But my tank top to bottom is 31" so I needed high power lighting to reach to the bottom. With all that lighting, my SPS are above halfway up the tank on several shelf rocks. It stays colorful, polyps are always open.
I run 4 4k gyres at 40-70% and they're about 3 " below the surface moving gentle currents of water over the SPS. I have two more MP40s pushing water from the back to the front to eliminate dead spots at the bottom. I also have one more powerhead rated at 6500 gph running near max pushing water around some of the SPS too. Plus 2400 Gph running out of my return nozzles which, blow gently over the SPS as well.
I think you're right in that your water parameters are near stable and in line with what they need to be. I've watched my acros grow from half inch frags to 8"+ monster colonies over the past 3-4 years (8 year old tank running).
That said, I still can't grow some SPS. I just added a Green slimer about the same level as the rest of my SPS on an empty rock and it's growing, but excruciatingly slowly. I'm hopeful over the next year or two it doubles / triples in size. It's not dying.
I think everyone is dead on with flow and lighting. I've never tested the par of my lights, but I assume my Acros have what they need at the levels they're at. My Acro / monti / Hydnophoria shelf.