I think the problem is that the binding only posts updates to the Item when the device is manually changed, but only Commands to a Group get propagated to all its members. events.log will confirm this.
Usually one would use a rule. But if the “the control is jittery” there when you send the command to the Group Item, it’s going to be jittery if linking the Channel to the Group Item worked too. The problem there is many technologies cannot handle sending out a whole bunch of commands to a whole bunch of devices all at once and that’s what sending a command to a Group Item does.
In those cases, you need to loop through the members of the Group and add a short delay before sending the next command. There is a Block library in openHAB Rules Tools [4.1.0.0;5.9.9.9] called Gatekeeper that will do this for you.
Beyond that, it might work to use the Follow profile on the members of the Group and link the controller to each light individually instead of the Group. The follow profile will command the Channel linked to the actual light based on updates from the controller’s Channel. But again, you’ll have to link the controller Channel to each light in addition to the Channel for the light itself and use the follow profile. And there is no guarantee this will work any less jittery if the number of lights increases beyond a certain point.