Simple answer- you can generally turn basic minor chords into minor 7ths, 9ths, and other permutations. With the major chord, you can often turn that into a major 7th or 9th.

You'll still need to see if the extended chords sound right, and the easiest way to do that is record your practice session on your phone or something and listen to it. It's hard to be objective while playing, especially at first when you have other things to think about, but when you're just listening you can hear all sorts of things you didn't pick up while you were playing.

To get a little deeper into this, chord symbols are there to give you the sound, the harmony of that point in the music. The way I look at it, there is no difference between a C minor triad and a Cm11th. They are both built on the sound of C minor, what flavor I want to play will depend on things in the music.

I'm not saying a person shouldn't know how to accurately spell chords because it's important to know that stuff, but it's just as important to play what goes best with the music at the time, as well as evoking the kind of mood you're looking for in your own performance.

As you observed, doing what you think sounds best is the right way to go, and you also mentioned not know how to sound "best" yet. If you try that tip and record your practice sessions and listen to them, no matter how painful it might be at times, you will improve twice as fast, if not faster. Why? We all know what good music sounds like, or at least what we like to hear. It follows that you'll want to play the music you like, so if you record/listen to your practice sessions, you'll immediately hear what you like or don't like and be able to make changes.

That's one of the reasons why people who record a lot are also often better knowing how to sound their best; they are always listening to their own playing.