I've been working at moon shots for some time using a Nikon 5500 with a Tamron 16-300 mm lens at 300mm. You need to have a good tripod, preferably weighted down to keep it steady and use a remote trigger to lessen any extra vibrations. If you do not have a tripod, try to find some solid surface like a fence post or such to rest or brace the camera on. I know that I won't even think about hand holding the camera (at my age, steadiness of holding anything is a challenge!). That being said, then set up a systematic series of shots, putting the camera on manual (Focus, fstop, shutter speed) maybe starting at an ISO of 100 or 200. I generally start at shutter speed of 1/100, f8 and ISO of 100 or 200, then changing the speed to 1/200, then 1/400 and even 1/800. I'll even then change the stop to f11, and again take a series of shots changing the shutter speed as indicated. You will have to check on this as I live in the Los Angeles area and if you are in an area that has clearer air, you will have to adjust the setting. Another factor is the focus. Put you lens at the maximum but then carefully check the focus. Many lenses will actually have their infinity focus just below the maximum physical limit of the lens so check it out, possibly using live view. Here is the best shot of a series of 12 that I took last nite. Usually I will have only a few shots (2 or 3) that will be acceptable out of 12 or 15! In any event, I have a great deal of fun doing this and have great satisfaction when I get a good shot.
Another couple of comments: for lenses less that 4 or 500 mm, you won't get a great deal of resolution of the craters. For me, with a 300mm lens, I get the best shots at the half moon stage and even then the craters come into view in only a portion of the shot. Full moon gives basically a circle with lots of gray shading and no real definition of the craters. And a good post processing program can really make a difference. I use Lightroom. Need to sharpen and crop!

Hope this gives you some guidance.

First, I'd suggest you stick with Nikon... But buy lenses instead of a camera.

Try one or two or a few fast primes.

You mention portraits and available light photography... primes can be one, two or even three stops or more faster than zooms. The larger max aperture both gives you more control over depth of field (especially important for portraits) as well as being able to shoot in lower light.

For portraiture I'd recommend 50mm and 85mm lenses which will behave as short telephotos on an APS-C camera like the D3100. For some things you might want a 28mm or 30mm "standard" lens. They can be used for portraits, too, but you have to be careful not to get too close because of perspective distortions.

An alternative might be a zoom such as a 24-70mm, covering much the same focal lengths. But most zooms are f2.8, at best... and those or any faster are big, heavy and expensive.

You also mention "nature photos", which in some cases means macro or close-ups. If interested in this, it can be done "on the cheap" by adding macro extension tubes (Kenko are very good) to any of the above lenses or the ones you already have... or buy a used macro lens. There are many vintage, manual focus that can do a great job. Some of those can be quite affordable.

Later if you still feel the need, you can upgrade the camera.