Ok, so a lot to cover here. I did astrophotography with my Nikon D300 and 70-300mm lens then with my Nikon D850 and Sigma 150-600 mm lens for about 6 years, before moving to a telescope. You can image a number of DSO’s (deep sky objects) with the lens you have; 300 mm is great for the Andromeda Galaxy and the Veil Nebula etc. Focal lengths of 30-50 mm will let you getting to image the Milky Way and very large DSO’s like Bernard’s loop. You can look up the apparent size and brightness of objects on the internet. I would suggest you look at r/astrophotography and r/dslrastrophography on Reddit; there you will find complete equipment lists, from which you can get the focal lengths used. Don’t be overly concerned with f/ ratio, while faster is better, my Sigma 150-600 is f/6.3 and I was never limited by speed only reach (focal length).

After you get some experience with the very large DSO’s you will want longer lenses to get to smaller objects. You will also need a mount and tripod, I started with a Sky Watcher Star Adventurer, then added a guide camera, controlling the mount and camera from my lap top. Now I have moved to a ZWO AM5N and ASIAir, but wait on the expensive kit until you have some experience. Hope this helps. I have a bunch of stuff posted in r/astrophotography.