Wood is already inert.
"...layer has to be a fungicide..." Any stock finish will do that. Bugs aren't interested in finished wood. There's no need for any chemical other than a regular finish.
Tung oil will waterproof wood. Properly applied, it soaks in about 1/4", preserves and seals wood. Keeps stains out too so any stain has to go on first.
Nothing I've ever heard of will fire proof wood though.