I have a Celestron 8SE with a ZWO ASI 224MC camera. Plate solving did not work until I put a 0.6 Focal Reducer in front of the camera.
Plate solving requires a FOV of at least .2 degrees. Your set up gives you a FOV of .14 x .1 degrees. With the .5x reducer you get a .28 x .21 degree FOV so that would work.
JohnD