I know in the turf industry, golf course maintenance, it’s common to top dress the turf with sand. Good for soil structure. Not sure if a layer if sand will help. The fungus and bugs are already there.
I have used diatomaceous earth and sand as a cover. I found it helped with fungus gnats, but a bit messy when watering. Covering drainage holes is great advice, gnats love em. I have experimented with using pure coir as a cover for soil, it tends to dry out pretty quick and keep the gnats at bay. My best luck against fungus gnats has been with mosquito dunks. I crumble some on top of the soil and I put one in my watering bucket. Fungus gnats never get very far past a few fliers in the tent.
This year I used hydroton clay pellets over my fox farm ocean forest based on the experience of another grower. Not only should it keep the top inch of soil moist for the roots to grow, it also seems to keep any fungus gnats away. I had zero gnats….I normally get at least 1 or 2 visible gnats. I keep some “Gnatrol” powder for a soil soak if I ever get an infestation, but FFOF has been pretty gnat-free for me.