A friend towed a 19ft cruisecraft up there with his pajero and had no problems getting across indian head/waddy. He said he just lowered the psi to 12 instead of the usual 20, chose 2nd low range, kept the revs up and he got through while others were getting bogged. His trailer was unmodified. Not sure if he lowered the trailer pressure to 12 as well. Probably a good idea to get the trailer foot bigger as well.
There is only a couple of sections. Eeither side of indian head and waddy, plus getting on and off the barge on the mainland side that is really bad. Around waddy itself has some pretty average sections as well, but not as bad as getting on and off the beach.
I would probably pump the tyres back to 20 for the main trip up and down the beach.