i Made tree straps exactly like those, out of nylon tubular webbing rated for 4000+ lbs.

the sewn loops didnt hold, well, not all of them have held.

I gave up using them, when a loop snapped, it was SCARY.

I sewed a 1.5 inch long 'X' shape, and boxed it, usiing a zig zag stitch, and non cotton thread.

IM not sure how I could have made it much stronger?
Do you mean like the slapstrap? If so this link has some tech info on the different ways these type of straps are sewn and why yours may have had problems.