Ok Progress.. Right clicking the Content.mgcb and picking MonoGame Pipeline Tool (Set as Default) solved the problem of the content manager not opening.
Seeing the file as text was actually useful since it made me realise that my customer processors where referenced using a drive letter that no longer exists in this Window 10 build. The Pipeline tool does now see my processors and some content builds.
Unfortunately most of it doesn't.
I picked a few content files at random and they still open just fine in 3DS Max 2019.
Seems to be the same error for all the content that doesn't build:
Processor 'EffectProcessor' had unexpected failure! System.DllNotFoundException: Unable to load DLL 'd3dcompiler_43.dll': The specified module could not be found. (Exception from HRESULT: 0x8007007E)
Maybe even if this stuff is obvious to some people maybe it'll help someone. Any help would be appreciated too