Jeff,
If I made the connections inside the access boxes I think sharkbites would be fine, although I'd probably still just use typical crimp rings. If I opt to bury I think sharkbites would bite it.
(I don't know if they are underground rated, but I suppose they could be)
Truthfully I'm a bit leery of any connection 6' underground. The hassle to dig up and repair is just too much risk. I'd much rather just bring them up to the near surface at those 5 locations and make the connections there. I do like how easy sharkbites are to replace/repair.
Len