I would put the straps through the loops of the bags on the right and secure it over the bags on the left. And vice-a versa for the other side. Stagger the three rows with no straps along the length of the trailer.
How about pallets on top, although easier said than done but sure you could find enough to cover some of the bags and get a tight hold of it?
i tried the no strap rule with bags of china clay.similar stuff…and on pallets…vosa didnt like it…so relieved me of £100…told me to strap every 2 bags except those behind the posts…so…if your ok for cash…dont strap em…we all know its pointless…we all know they wont move…its all a fruitless exercise…but vosa has their rules…they abide by them…not make them…and they say…all loads have to be strapped…even fragile ones…even if you damage the goods…i say…packaging is the problem…no one enforces that its safe to transport…except adr in my opinion.