Issues after PCIe SSD upgrade

I dismantled my iMac 19,1 (2019, 27") and replaced the 32GB PCIe SSD with a 512GB one from another iMac model. I reassembled the iMac and used duct tape to hold the screen in place whilst I tested the Mac to make sure it all worked correctly. I also fitted a new video cable.

In a nutshell, everything works except the display (black, nothing on screen when using flashlight, etc). Using an external Thunderbolt Display I used Terminal to reset the Fusion Drive, and installed MacOS Sequoia. After running Apple Diagnostics I received a message that there were issues with both the SMC and Display.

At this point I can use the machine with an external display. I am waiting for the in-line thermal sensor to arrive, which I assume will resolve the noisy fan (although I have temporarily used Macs Fan Control for that).

I suppose my main question now is whilst I wait for the in-line thermal sensor, does the lack of it on a third-party HDD affect the SMC in some way, and subsequently the power to display? If not, where should I start looking for display faults? So far I have checked the capacitors and fuses on the Logic Board and tested the video cable - all looks fine. Also, diagnostic LEDs 1 - 3 and 5 are all green, 4 is off.

Edit: As additional info, I also noted that in Activity Monitor the CPU load is significatly hugher than it used to be, average reads System: high 60s to low 70s (where it previously read around 10$ when idle.