How stable is the trailer left/right at the jack? With the jack extended that high, I'd want the wheels really well chocked (not with the X-chock type). With that piece of 4x4 (or 6x6?) you have running in a fore/aft direction, I'd turn it 90 degrees so it points left/right so that way if the trailer moves laterally (left or right) for any reason, the jack won't potentially roll off the 4x4 piece. If there is some wiggle in the jack at that height, it is possible to add a plate to the underside of the A-frame with a hole in it to better stabilize the jack.
Engineering-wise, the stack of wood would be the most stable if it were pyramid shape at 45 degrees on the sides instead of stacks of wood the same width from top to bottom. If the stack of wood were say 2' tall, you want the bottom to be 4' x 4'.
I once had two levels of 6x6 blocks under our jack in a CG because of the sloping site. Unbeknownst to me, it was at the beginning of the season and the ground was saturated plus they had put down new gravel without compacting it. The blocks sank into the ground towards one side and the TT lurched to the side and forwards a bit. Luckily the truck was still chained up to the TT. No damage, just a PITA to get it jacked back up. Now I pay close attention to the ground conditions, have longer pieces of 6x6 and orient the top piece left/right.
FWIW, with a TT that nice looking, I would get a nice electric jack (ball screw type)... :)
