Keep the power on when turning. if you have to slow down don't take your foot off until the wheels are straight and the back wheels are in the front wheel tracks.
385/65 sounds like a pretty low profile tyre to me which will give much less benefit from de-inflation than a higher profile.
A standard 11R22.5 has a profile of 88% and I would put money on it getting further than the wide 65% profile when both are deflated for maximum sand traction. Also much less stress on the steering during normal road driving.
Also when turning around go high on the beach first then slowly turn down towards the water and harder sand.
Measuring 'slippage' is not that hard. When I studied tractor tyres and how to make them work some 30+ years ago now We would put a chalk mark on the drive tyre/s and walk beside the tractor when it is under load and drop a stick in the ground as we go. Measure the distance between the sicks and know the rolling circumferrance of the tyre and do the maths and as stated look for that majic 10%. Adjusting tyre pressure is how you adjust the slippage and the other thing you should see when it is at it's maximum is the sidewall distortion of the tyre. Too much here will destroy your tyre and none means your not set up right.
I put the Cropmaster on a tractor sled to see how it would work. One of the guys asked me how much water was in my tyres and I said not a drop. When I watched the short video there was just the right amount of distortion and an unmeasured amount of slippage was noticeable. I did prove that there was not water I stood right infront of the valve when it was at the bottom and pressed, I stayed dry.
He is working in the Mallee where there isn't any hard wet sand. I had a week in the desert parks in my OKA a couple of years ago and lots and lots of soft sand like Bondi Beach. When you are on a soft sand track you really do not have the luxury of wide turns and picking your ground but have to deal with the ground presented.