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@Damien I'm afraid that at a focal length of 600mm imaging deep sky objects you are very soon going to have to autoguide.
The D750 has 6um pixels. At 600mm focal length that is 2 arcseconds per pixel. And your mount will typically have a periodic error of 60 arcseconds peak to peak. It has a worm period of around 10 minutes (638 seconds to be more precise). So every 5 minutes or so the mount drifts around 30 pixels in Right Ascension then back the other way for another 5 minutes.
Until you start autoguiding you would do better imaging at 200mm focal length or less and keep your exposures short.
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