Gather a close circle of trusted friends and let them listen to your works, in person if possible. Honestly, these are pretty much the only opinions I truly value.

I always have the Brainworx bx_meter sitting on the master bus, calibrated to K-20 - by the mixing stage at the latest. Headroom problems are literally non-existent, as the average levels dance around -20 on the digital scale, leaving more than enough headroom for random peaks

Squeeze the most out of one sound. Use filters, lfos, delays, etc., to create an interesting sound instead of 3 semi-interesting sounds. Might also want to ask yourself the question: is layering always necessary?

If you only start producing a few months ago...

What he said... Though, a decent high pass filter on any channel needing some low rumble/mud removal is a great place to start, technique-wise.

Mixing is definitely a separate process for me. Of course, "mixing" happens while doing sound design stuff and writing, but I like to give myself a few days to focus on mixing 100% after the track is done.

I like to find a comfortable level using the volume knob on my interface, where all instruments are clearly heard, and simply recall that setting as my default working level. I do, however, change the level quite a lot but can always come back to the default value by the push of a button. Convenient and it works. And yeah, I don't really care about any standardized monitoring levels, not in my current room at least. It's good to check at really loud levels for any harshness in the high-mids and highs, as I've often had to get on with some de-essing action after doing so.