This will sound weird --- I had a similar problem with a Celestron 8”. I solved by unscrewing the “T” adapter. You will likely see that a cavity exists between the telescope body and the “T” mount. You can, with a little creative thinking, mount a supplemental lens in this hollow.

We commonly attach a Barlow lens between eyepiece and eyepiece tube. The Barlow is a negative power lens system that elongates the focal length of the objective lens. Thus the Barlow adds magnification. Again the Barlow is a negative power lens.

Now reading glasses you can buy at the drugstore are positive lenses. So are the close-up lenses we use on our cameras to make macro pictures when our camera won’t allow close focusing. If you install a positive lens between the camera and the telescope body, you will be shorting the focal length and the image seen by the camera will shrink.

Go to the drugstore and buy some inexpensive reading glasses like +2 or +3 or +4. Hold one of the lenses between your “T” adapter and the telescope body. I will bet the +3 does the trick. Now measure the cavity and see if camera close-up lenses (some say filters) are available for this diameter. Experiment with reading glass lenses. If you find a + power lens that works, this tell you the power you need.

An eyeglass optical shop can cut eyeglass lenses to fit. You might find that such lenses are already available, check your telescope accessory sources.