The general rule of thumb for fresh manure in garden applications is 90 days to allow any pathogens to be consumed by the "good bugs". I use a deep litter system in coop and run. When I clean litter out of the coop, it gets shoved into the run, where the chickens mix it with leaves, grass clippings and garden debris. After 3 years of management, my 500 s.f. run is finally covered with a nice soft 6" layer of deep litter. I can access this any time I want to provide compost for gardens and orchard.
Your burial system is fine. But, I suggest that you not put it in your garden unless it is buried quite a distance from any root zones. If deep litter does not appeal to you, I suggest that you do a drum composter, or build a compost bin and keep a bale of hay or straw handy so you can layer your compost as you build it. Beware, some pastures are managed with herbicides that remain potent even after the hay has been harvested, fed to the cow, passed through the cow's gut, and the resulting manure turned into compost. Unsuspecting gardeners have had their gardens ruined by using compost that is tainted with some of these newer herbicides. As for straw: most grain crops are treated with glyphosphate prior to harvest to cause all of the grains to ripen at the same time. Not something I'd want to bring onto my property. Just know where your material is sourced from, and how it's been handled!!!