Be sure to check out the RELPOSNED message on the ZED-F9P. RELative POSition North East Down, gives you the relative position of the Rover compared to its Base. In a “Moving Base” scenario, this can provide excellent relative positioning, but not absolute. For absolute accuracy, the moving base needs its own corrections, from a local fixed base.
Another user was trying to do (almost) the same thing here. Their application was drone orientation, compared to a moving vehicle. But the principles are the same.