Not trying to be a smarta$$ but since you didn't list it in your steps, I have to ask. Did you flash the full Sept image? IMHO, if you are going to download the full image each month or whenever, and extract the boot image to be patched, might as well just flash the full image then flash the Magisk Patched boot img instead of trying to get the OTA. If you want to keep your data each time you flash the full image, just remove the "-w" in the Flash-All.bat file.
I did flash the full image. I caught the "-w" tag and realized I wasn't going to delete any of my data.
I guess next, I can try with the OTA file, but I don't know how that's going to be any better.