The timing of your question is fortunate, as I recently read an article about ventilation ducts, their size and the conditions inside them and so can understand what you want to achieve. :) (The article was making the point that in most cases, people cannot crawl through ducts like they do in movies and videogames).
In this case, the pairing of a T265 and a depth camera like the D435 may be ideal. The T265 can do the SLAM mapping, and the depth camera could use obstacle avoidance to autonomously steer the rover around obstacles in its path. Intel published a lengthy presentation on YouTube about doing this. The part about using D435 starts at 17 minutes 15 seconds into the video.
https://www.youtube.com/watch?v=62vm0_RZ1nU
You could probably do it without the D435 though if you just set the SLAM software to live-recording mode and let the robot patrol the ducts until it has built up enough detail on the SLAM map for your satisfaction.
If you do decide to add a depth camera, you do not need one with an IMU like the D435i, as the T265 has the same IMU component as the D435i inside it already. The D415 may be a good choice, as its smaller field of view will not matter in this small duct and it has around 2.5x the image quality of the D435.
