You have everything you need inside of Renoise already to both mix and master. So if your Linux build can run Renoise, you’re off to the races.
No need for fancy plugins. They won’t make or break your mix/master.
The problem is more outside of the box. What does your room sound like, is it treated? What are its flaws? I 'm talking about frequency bumps/holes. Followed by the accuracy of your monitors. And are those a good fit for the room you are in?
But above all else it is how developed are your ears? I’m not talking about your hearing range. Though that has something to do with it as well, of course. But can you hear the problems? It takes skill & practice. And honestly, not everyone can.
But before you get to mastering your stuff, you need to get your mix right. Which is more a matter of arrangement than anything else. Anyway, see this excellent thread for advice on mixing.