X-Box GamerTag Transfer

When I originally set up my son's XBox, I used my e-mail address rather than setting up a GamerTag for him in Family Sharing attached to his e-mail address. I would now like to limit his screen time, but since the GamerTag he uses is linked to my e-mail address and I am the administrator, it seems there isn't an option to do that (only for Family Members). I thought I could transfer the GamerTag he uses to his e-mail address (from mine), so he can retain his levels, points and purchases rather than starting afresh and losing everything, but Microsoft said that if I would do that, my e-mail address would not only be removed from the XBox, but deleted altogether as if it never existed on any device, MS Office and personal Outlook files etc.

There must be a way to either limit screen time for everyone (including the administrator) on an XBox, or to switch a GamerTag profile to a different e-mail address, without the original e-mail address and everything associated with it from being deleted as if it had never existed?

Many thanks.

:)