Hi Amber,
A couple thoughts come to mind. I am visiting my family in FL this week and have been having conversations with them about building their soil. They are just starting the process of thinking about growing a garden and have not done so before.
The issue with your soil washing away into the sand... Have you considered or used any kind of mulch on top of your soil? Leaves, wood chips, or dried grass clippings could help distribute and soak up the
water when it rains so that there is less to wash away the soil into the sand. Combine that with Michael's recommendation of sheet mulching and you have yourself a solid container garden that should hold up any rain.
If you were to try the
hugelkultur method I would recommend wood chips to fill the voids of the branches and/or sheets of
straw to cover them so the soil has something to sit on, this way it does not fall into the cracks.
I did not want to hijack your post so I started my own on hugelkultur with questions about doing it in NW Florida.
https://permies.com/t/23280/hugelkultur/Hugelkultur-Florida#188202
There may be responses that get posted there which would also be helpful to you.