Updateing from OC 6.0 to 8.5

Hey,

Iam currently running Catalina with OC 6.0. It works perfectly but I want to update now to at least BigSur. I tried to update my plist with ocat and booted it from a stick. Sadly I had no luck. Got OCB StartImage failed which is strange because Windows is ona completly different SSD. Anyway I thought it would be much better to start from scratch again and build the plist from zero. If I start from zero and build the EFI with OC 8.5 will it work with my current installation? Or should I start from complete zero and also reinstall macos?

Or is there a much easier way to upgrade from OC 6.0 to 8.5 ?