I imagine I could control 4 corners without much extra effort or cost, the program would be the same front/rear; just a copy/paste.
I bought my rear air springs on eBay, one was $280 with 63k miles, the other was $236 with 45k miles. I got them for ~ $100 cheaper than the auction listing by making an offer. There were cheaper ones out there, but I wanted ones with low miles. Everything else I bought new (lines, fittings, compressor, valve bank, PLC, power supplies...) I'm using the stock VW valve bank.
My rough calculations:
Assuming ball to chassis = 1 foot, the moment from 500lbs on the ball = -500 ft*lbs.
Now with a WD hitch w. 3 foot long spring bars, the moment is (390lbs * 4') = +1500 ft*lbs
Net moment is 1000 ft*lbs vs. 500 ft*lbs.
Again, real rough calcs but to me even under the most optimistic circumstance (just looking at static loading on a level road) the moment using a WD hitch is 2x. Once you factor in dynamic loading like going across a dip in the road or up a steep driveway (where trailer and vehicle make a V), the WD moment is probably much higher.
I understand. I’d love to put air suspension on all 4 corners. The controls were my worry especially towing with WD hitch (auto level and such).
As to the hitch moment. The WD reduces the moment under normal conditions compared to non WD hitch. I think VW was remis allowing a 600 lb tongue weight on non airbag equipped vehicles (with non WD hitch). I suppose some sag is normal.
On a different note. Where did you buy the air shocks and how much were they?
Does your system work with the stock ride height valve?