Gotta read this later on computer, cause I have no idea what you mean by deploying system image (edit: I think we just use different terms). I have tested to flash system image using both fastboot and twrp, but device will never reboot.
I have now played with this and flashed stock rom back 4 times, it flashes Oreo if I remember right, so it takes ages to install those updates again. And installing that unofficial lineage os 19.0 will fsil at error 28 at twrp if partitions not resized and doing that will format those partitions and stock vendor is then lost and replaced eith somehow modified version from that rom…
Cause of sll this, I wont test sim yet, cause now back st stock. I might test to flash these later again, but would prefer waulyvto flash over stock.
In meantime, keep on eye also Nokia 7 Plus request thread