2 - Go back to Device Manager - Bluetooth - I have found that (at least on my Surface Laptop 3) that the only Bluetooth Device that has an option to alter the Power Management setting is the Qualcomm Atheros QCA61x4 Bluetooth - click that and uncheck the Allow the computer to turn off this device to save poweroption.
So far so good. Hope it works for you.
It completely stopped disconnecting, works like a charm.
I thought this option might have to be unchecked somewhere else as well, but didn't look hard enough, it seems!
You are a wonderful person, Mike!
And if anyone from Microsoft reads this - there's the bug, the problem was fixed for anyone doing this.