" Bought side fillers but they don’t go over the back plate cover. "

The side fillers are filling the gap on the side as you noted but if the gap is there and equal on both sides the hole was not cut right, too big.

You need the backguard. That is an island trim and can not be used against a wall, because it won’t meet clearance to combustibles. And it isn’t installed correctly.

What do you mean ? Island trim?

I think Lomo means that the range you selected was meant to be installed in an island, not against a wall. As I understand (check your range install instructions to be sure), a non-combustible material must extend below the countertop.