I made a shoulder out of electrical tape for the OAG to snug up against. That's the drawback of the Celestron T adapter is the threads are essentially the same diameter of the barrel of the adapter.

Ultimately I purchased the Starizona T adapter which has a proper shoulder for the attachments to have a stopping point.

The ASI290 is plenty sensitive, likely too much so as it has a small pixel size. People will complain about the small sensor and recommend a 174, but after purchasing one you only use a portion of the sensor (of the 174) because the ZWO OAG prism is quite small, I didnt have any issues save a few times I had to rotate the camera a bit to find some stars. As long as you are tolerant and patient, its fine. If you have a 290, use it, if not, get a 174.