I’ve seen your other post where you already tried using physics constraints. Using target orientation or position of the constraint is the way to go when it comes to controls. The jittering and parts getting disconnected is something normal for PhysX. You could minimize it by setting more position and velocity iterations on parts in properties, enabling projection on physics constraints and tweaking mass of the parts - there shouldn’t be more that 1 to 10 mass difference between constrained parts. Most likely you will still have problems when trying to lean on arm against the ground. More stable solution would be to use Articulations as I’ve mentioned in previous post.
There is something else you can try too. I’ve exposed function to BP which allows to set Inertia tensor of the simulated body. It can help with stability if you set a much smaller inertia on axis that should be mobile and very high inertia on axis that should be locked. There is an example of using this function on propeller, in Simple Examples map.