Love your ducks. Been ages since we had a few here.
If the sand is working for you in your environment that is all that matters honestly… there are lots of preferences to bedding on all poultry here and mostly that is dependent on environment climate issues, available materials and what keepers of any species finds works for them or even is cultural for them.
Since you have bugs, if it was me I would clean out the house good, good time to wash it down, sanitize and treat for bugs, Perfume it (aka treat the housing by burn smoldering certain herbs… hippy me) I would put the old sand out to be recycled into garden soil or potting mix… I would treat their housing floor and walls with something I am comfortable with. Again we are all different. I tend toward the hippy dippy stuff but not 100%… if I think it is modern science or chemical time problem, then I science the heck out of it... just depends on the issue. Then put all new sand in it.
I would also carefully inspect the birds for parasites and treat as needed. Might even give them a medicinal bath (homemade tea bath)… may as well.