Just a quick reminder of how my issue worked out - for new readers of these posts.
I had the exact same problem others are reporting:
Keyboard disconnecting, needing to be removed and re-paired in Bluetooth settings
The fix for me was to install the drivers for the USB motherboard chip.
The drivers that came built-in to Windows 10 could not be tweaked to work - many, many attempts.
So - visited the support page for my motherboard, (ASUS) and downloaded the driver that they supplied for the USB3 chip.
Ever since that got installed the keyboard has worked perfectly. It NEVER disconnects now.
This fix has survived 2 Windows 10 major updates (W10 2004 and W10 2009).
Hope this helps someone.