Way too many maybes at this point.
Try to eliminate a few basics... On flat ground hold the front brake LOCKED, and try moving the bike froward, and backward with some real force. Quick strong snatches. What you are looking for is an play in the steering tube. Maybe you will feel a deflection or shifting of the bars.
Next on the center stand hold the wheels (one at a time) top, and bottom and try to wiggle them. You are looking for either worn rear gear case bearings or worn front axle bearings. Again strong quick jiggling will show up as play or clicking.
While still on the center stand try to "twist" the engine. There is ONE center bolt or pivot bolt thru the frame, and front of the engine. The bushings that isolate the engine from the frame are prone to wear, and will feel like a shudder under certain conditions. The whole assembly is kinda heavy so some creative angles, and real force is needed. Similar test for the rear shocks. Try to move the mounting points to see if a bushing is worn. with the bike flat on the tires you can try to lift the rear body, and see if you feel some odd movement or clunking from the shocks.
Most odd shudders happen in the suspension. Some come from the clutch slipping, and some come from worn variator parts. It never really comes from the belt unless it is about to come apart, and that will be obvious once you open the case.
If you still need help after checking these things there are a lots of people here to walk you thru the CVT parts, and how to improve the "feel" of the bike. DeGlazing the clutch bell w/ emery paper/changing worn clutch springs ect.. Rollers that have worn flat spots are another culprit, and fairly easy to replace.
Try the punch list for peace of mind... It may be something there.
