The problems you're mentioning about the volume sounding different in the mix/master compared to export sound like some kind of issue with your audio driver. I would check and make sure no enhancements are on, especially if you have a Realtek driver. I used to have to reinstall HD Audio driver for Windows and Reaktek alternately in order to get a clean, uncompressed output.
The good news is despite this frustration your mix sounds pretty darn good to my ears. The sub is nice and thicc. Could be louder at points due the open nature of your mix (and I think my headphones emphasize bass frequencies just slightly.
Things I would see after -- this mix is so close to perfect and yet also so minimal that little things like the panning of your aux elements and leads become very noticeable. Even the dryness of different instruments is front and center. So I would just take some time to adjust FX/pan here and there and make things a little more dynamic. Beyond that, great work!
9.1/10