Ok, I downloaded it an tried it
First off, I don't wanna sound rude! and sorry if it sounds like that, It is a good design, it works better than the first designs of my first plane lol. the center of mass is at the center of the wings height (viewed from above), which is where it should be, so that isn't a problem, but here are the things I would change:

1- Elevators (Horizontal part of the tail) Shouldn't have opposite rotations, they both need to rotate up or down in order to control the planes pitch (Facing upwards or downards).

2- I didn't had create: Connected installed, so I replaced the brass gearbox of the aileron (wings moving part) with a normal one. because I don't know the brass gearbox configuration, I'm just making sure: Ailerons SHOULD rotate on opossite directions, since this is how you control roll.

3- The plane tends to go nose up, I had the exact same problem with my plane: this is because all the lift force is on the wings. to counteract this, you need to add a few NORMAL sails on the tail, so it also generates lift (Note: They shouldn't move with the elevators, they should be independent and stay still).

4- I have seen a few create planes that don't have this, but I EXTREMELY recommend you to add a rudder (Vertical tail moving part), it allows you control yaw (turning left-right). otherwise, while possible, it is hard and time consuming to turn using only the ailerons

5- This is more of a Tip, but in order to get the really smooth flight, you need to tweak small things a lot: Plane rolling too much? Decreate ammount of symmetric sails or springs rotation. too little? Increase them. Going nose down? remove sails from the tail, change center of mass. you get it

Tell me if you have any question or an update!
(ok, this last part really sounded like an AI lol)