DISM throwing 0x80004005 error, apps not opening, W10 fresh license ke

Hello everyone this is my first post! I appreciate your patience in delivering as much info as possible.

Windows 10 Enterprise Ver 1803 OS build 17134.1130

Here's the rundown of the issue, history, and what I've noticed.

I've recently acquired this PC from a workmate and used it for in excess of a year with zero issues. He suspects the Enterprise key he used for this version of Windows is either pirated or just grabbed from the internet, but worked at the time. There was a KMS program installed to prevent windows from checking against this key. I left the PC alone for 10 days while on holiday beginning Dec 11th 2019, and on coming home it I've got an 'activate windows' in the bottom right, and loads of strange issues going on. I suspect an Windows security update that had finally smacked-down on the KMS utility and barred it.

I was quick to remove the KMSpico utility after discovering it was used, and after attempting to do a check of outbound rules just in case in Advanced firewall, it wouldn't open. Got my legitimate key, went to open Settings, the 'activation' tab doesn't open when clicked. Sometimes settings doesn't open all together. Updates will be stuck on pending download. Sporadic behaviour.

Network and sharing center, System, Power options, even Acrobat 12. the list goes on- all Non openers.

Ran a sfc/ scannow, couldn't repair some files. Ran DISM /Online /Cleanup-Image /CheckHealth, throws a 0x80004005 and wont start. When trying to run some Windows troubleshooters, same error code weirdly.

Sometimes when trying to Activate windows, or perform other menial tasks, I'll get nothing then after a minute or so, a Windows Script Host dialogue box with either a 0x80041032 or 0xC0020036 error 'running Windows non-core edition, run slui.exe etc.'

It hit me after all this the Windows key I bought was for Pro, so that's another issue in the mix. Tried the Windows 10 Update Assistant (as if that would actually solve things) which downloaded the update, but failed the install.

What's my situation like right now? What other checks can I perform to test integrity because something is definetely amiss! What can I proactively do next?

Regards
Richard