Sorry to be a pain, but are the late December orders looking to be back on track, or can we still expect a delay
I had a similar issue with my gigabyte x570 board in a different case. Assuming the b550 is the same, seems like the backplate is part of the cooling soln for the vrms, so you can't just just get rid of it, however you can try doing what I did and route the riser under the backplate (either completely under or out the cutout in the middle. It's held on by a few tiny screws on the back, plus one tiny screw on the top of the board in the corner. Also be careful to note the position of the thermal pad.