Ahh, you want to open that ol can of worms, hmm?
The topic is super-broad as well as very deep so we need to set some boundaries for the scope of discussion, don’t you think?
I can tell you this much. About 15-18 years ago I first worked in a “professional” recording studio as an intern (mostly recording dialogue and radio commercials tho) and the main room in that place had decent acoustics and big 3-way active monitors, and now looking back I think that was the biggest difference of that place compared to most project studios. Its that simple.
If your listening environment is subpar, you will most likely find yourself in a position where no amount of “mixing” is giving you a great sound off the bat… You find yourself listening to the mixes in different places with different systems and little by little make corrections that make stuff sound more balanced across them…a soul destroying, if not sobering experience.
So the no 1 priorii IME is, if you’re not happy with your mixes, check your listening environment. Is there something you can do to improve your acoustics? Are your monitors placed optimally? That sort of thing. This doesn’t need to be expensive or fancy, carpets, sofas and thick, heavy curtains (but no egg cartons ffs!), speaker stands, that sort of things can do a lot already.
This is the most vital piece of the puzzle. Without this in place, nothing will ever make sense.
In a good listening environment for audio work, you find yourself doing the right things instinctively, just by following your ears.