Windows 2019 WSUS Error 0x8000ffff when pushing out Windows 10 Upgrades

I've had this error on 3 separate Windows 2019 servers I've tried to install Windows Server update Services (WSUS) on; It's worked fine on every Windows 2016 server I've configured, so I'm assuming I'm getting something wrong, or something's changed in 2019, but I've had no luck finding it:

Configuring WSUS on 2019 using the defaults leads to a WSUS Server that can push out all normal security updates to Windows 10 Clients, but fails on the upgrades category with Error 0x8000ffff.

This was a fairly common issue on the 2012/2012R2 version of WSUS to do with Upgrades being Encrypted. The fix was Add the ESD MIME Type into IIS

Out of the Box, Rather than have no ESD MIME Type, 2019 WSUS Adds Application/vnd.ms-cab-compressed. I've tried leaving it that tried changing it to application/octet-stream, but on all three servers, Clients refuse to Upgrade to 1903 with error 0x8000ffff:

I've tried googling around this, but most things I've found point to deleted KB3194588, or seems to be specific to Windows 2012

Can anyone point me in the right direction? I don't mind completely wiping the WSUS Installation on the current 2019 server I'm working on, if that's a quicker way to get upgrade packages working.