This is probably the most important advice I will ever give you: Don’t rely on mixing audio, get it right in MIDI first.
It’s hard to admit, because your composition is so well-crafted and sounds so good already and you like it and you are used to listening to it on your headphones that you always use… and yet it doesn’t sound good on the HiFi in the living room or on the smart phone. Chances are that you didn’t get it as right as you thought you would. And you’re also very tired of it. You want to finish the project, so you told yourself: I’m finished composing, all the notes are in place, it’s perfect, now it’s just a little mastering, EQ, reverb tuning.
I know what I’m talking about because I myself had a hard time admitting that at least 75% of all deficiencies in my mixes were something that could have and should have been fixed in the MIDI. The majority of mixing problems are instruments being too loud or too silent. You can fix that in MIDI and you should, because that’s where it’s easy. Balance out the instruments against each other in the MIDI. You don’t need automation tracks there, you simply adjust expression or volume or whatever you use. Do it there and you save valuable time later. Why? Because later you don’t want to make tiny, little changes to individual instruments in tiny little spots, but instead shape the overall sound.
My approach: I try to get the volume balance right in the MIDI stage. I use Expression control, I don’t use Main Volume at all. I try to start with an expression value of 100, which gives me room to make the instrument louder or less loud, and from there I adjust loudness as needed.