Thanks for your detailed response - I just have to mention how absurd and unacceptable it is that users should have to jump through hoops like this in order to fix the messes that Microsoft inflicts on its users, with no response or reaction from them to the issue.
Unfortunately none of this worked for me and I have tried all your recommendations, as well as several others recommended by others, for hours. My 'Privacy' section in Explorer Options has the 'Show recommended section' greyed out so it can be neither checked nor unchecked.

The 'Open File Explorer to' does not have a 'Quick Access' option so this has obviously been tampered with at Microsoft, for who knows what good reason. I cannot even pin a foder to the Start menu - trying to do this simply pins the entire File Explorer to the Start menu.
The most frustrating part of all these things (and this is not the first time by any means) is when the so-called Microsoft Experts post solutions in the forums (such as this one) that simply do not work or do not even exist. While Microsoft itself sits in its ivory tower and profoundly ignores everybody.