Hey, you're my kind of fellow. You'll have a blast on that little thing. You'll get very competant in no time.
I had a similar sand experience not long ago in Arizona aboard my Yam 250 wrr. I was trying to scout Camp Creek Wash, which is a natural drainage 13 miles long. Like your wash, it didn't seem to have a bottom. I tried it from both ends and failed after maybe a mile each time. Like you I came here for some pointers: up on the pegs, weight back, grip the seat/tank with your knees, arms loose, stay on the gas and go fast enough for the tires to get on top...
No luck so I mounted some new Michelin T63 DOT knobbies and went at the wash again. Presto! End to end in under an hour even with stops. At first I was going only about 15mph and a kid came whizzing by on a dirt bike doing about 30mph. I bumped it up to about 20 and it was much better.
You'll soon discover those rock gardens can be rolled well with a small bike. You just can't stop and your low gear has to be low enough to ride it slow.
I would have NEVER tried that last water crossing. Bold soul, you!
PS: Soon as your oem tires are done, get some T63's. They are cheap and work well in both sand and mud. They make learning a lot easier I found.