The Lil Wil magnet does
not separate from the coax.
I routed my cable out throut the firewall, along the windshield (under the decorative trim), and along the roof rack to the Mag base.
When I am not using the CB, I remove the whip from the base, and store it in the vehicle. But I leave the base, where you see it in the image below.
You can see it mounted here, in front of my wind deflectors.
I know you said you are looking for a Mag Mount; but you also say you want to leave the wire in place, and remove the mount. This sounds more like a permanent mount system.