That's why its important to continuously have a reference track handy to remember the goal for the type of song you are aiming for, even the worst of mixes start sounding good when there is no comparison. As long as you are refining with a good mindset and not one of tiredness and sloppiness in the ears you should be okay. Remember difference between mixing and mastering too, at some point you send it over to a different engineer, your goal isn't to make the track super loud but making it concise enough to where the mastering engineer can finish it into a good song. You want to work on not having a too crazy dynamic range. Also remember professional mixes have multiple mixers, to fill out the mistakes of the other, so take a break to where you can honestly judge the mistakes of your past self without fighting your ego. That way a concise mix is bound to happen. Lastly, really, create your own work flow, you will be stuck tired and de-motivated when you have no idea what to do next and there are a 100 options, you will keep on tweaking infinitely till you will start grazing through the line of overmixing.

I simply think a perfect mix is impossible to achieve. How could I know that I made all the optimal moves, and what to even consider optimal?

Things that are perfect are boring. Yes you want to know the rules but after that, after no improvements are possible , break the rules and make it different.