The night sky is a whole world above your head, and for me sharing it with others is one of the most rewarding things in life. A great viewing tool for the night sky is binoculars on a tripod, telescopes are too 'zoomed' for many objects and there's a lot more detail up there than most people are aware of, bins are excellent. But even with the naked eye; if you dark adapt your eyes properly on a moonless night you will notice all sorts of interesting groupings and blurry objects that make you want to know more; and moving objects too (satellites etc), always something new to see, always breathtaking; but even more so if you understand what those objects are!
Focus wise I am in the same situation with my 70-300mm, using it for the first time without auto-focus since the D3200 has no motors, that makes me want to upgrade asap. Like my lighthouse/Moon shots a few days ago, I used F9 because I was struggling to get perfect focus and it made it more forgiving, but it's always a compromise with shutter speed. One very useful accessory would be a cheap 2x viewfinder adapter, this fits onto the normal viewfinder but giving you some magnification and assistance with manual focussing; particularly when you need to focus on a star for doing star-trails for example.
Astrophotography really has very little to do with 'normal' photography, requiring motorised telescopes; light-weight CCD/CMOS imagers, narrowband filters, and software stacking etc. I did see your stacked flower picture which was very good! I assume you were doing something similar, but certainly to get proper images of the Moon or deep sky objects such as galaxies and nebulae then you would use specialist equipment - that doesn't necessarily mean expensive, but its certainly going to involve a computer; and leaving the DSLR free to do something like a star trail!
For a DSLR I'd say the best application is star trails: on a moonless night; put the camera on a tripod with a wide-angle lens like the 18-55, minimum f-stop, zoom in and focus on the brightest star you can see, pull back and compose your shot, then let the camera expose for 10 minutes or more .. ideally 30 mins or more, depending on light pollution, and also you have to watch for dew forming on the lens; I have a heater to stop that .. Onboard NR is fine; but you need a full battery as it will take 30 mins to do with a 30min shot. Actually harder is getting a good night sky image without trailing; I found I can get away with 7 seconds at 18mm; then boost the exposure with raw tools - I'm yet to get a 'good' non-trailed shot of the Milky Way. But its mostly about your choice of location; foreground objects; constellations visible etc, endless creative options : )
(A few words?! ^ Nevermind, get out there people! )