Residuals are the real key here. The same with getting rid of roaches. Newer generations of neonic insecticides (very safe indoors, no risk to bees) are slow acting, non-repelling and are extremely effective at knocking down populations pretty quickly. The critters will pick it up and spread it to wherever they are hiding. Combine this with steam cleaning, and it should severely knock down moderate infestations within a week and eliminate them in a few weeks.
I honestly wouldn’t even bother with DE. It’s messy and may not be great to breathe in, especially for pets. Its a decent physical barrier but there’s better ways to do that IMO.