I have also been aiming to achieve the same thing of a cage for harness whilst retaining the stock roof and hoops. After hours of research I've stumbled across this build which removed the hoops but used their location for the cage mounting points so retains the roof.
I wonder if it is possible to fix a bar behind the seats running width ways through the cabin for harnesses to mount to without the need for a full cage. Not exactly what you want but closer, link here: https://jr-wheels.com/vehicle-gallery/1054


The only other option I have seen is the post on bimmerpost: https://www.zpost.com/forums/showthread.php?t=1722062


I wonder if it is possible to fix a bar behind the seats running width ways through the cabin for harnesses to mount to without the need for a full cage. Not exactly what you want but closer, link here: https://jr-wheels.com/vehicle-gallery/1054


The only other option I have seen is the post on bimmerpost: https://www.zpost.com/forums/showthread.php?t=1722062

