Thunderbolt Ports Not Working - iMac Late 2013 Mojave

I recently upgraded to Mac OS Mojave, and it seems that my Thunderbolt ports are not working properly. I have a thunderbolt drive (self-powered) and an external display (connected by a thunderbolt to HDMI adapter).

Since the update it seems my drive won't be recognised (it is recognised when I connect via USB) and my external display is powered up but not connecting.

What is interesting is that the drive recognises that it is connected by Thunderbolt as it spins up, but the drive never appears on my desktop. The machine also recognises that something is happening when I connect my display, as my iMac screen dips to black for a moment as it usually does when I connect the monitor.

Things I've attempted:

• swapping ports
• using different cables
• Resetting the SMC
• Restarting in safe mode
• clearing the PRAM (twice)

The Thunderbolt bus registers 'no device connected' even though devices are being connected and the machine is reacting as I connect them:

It doesn't seem that there is anywhere I am able to download an update to the Thunderbolt Bus firmware either.

I can almost guarantee the answer will be 'you'll need to replace the logic board' but this looks like a software/firmware issue to me so I thought I'd see if anyone else has had a similar issue.

System: iMac14,2 Late 2013

OS: Mojave 10.4.3