Don't stain it at all. Use an oil-based finish and it will mellow over a year or so into a warm amber. Not too dark.

before you repaint, try glazing your mustard with water-based faux glaze mixed with raw umber-maybe about as dark as a light chocolate milk. Try it on a small area like between a window and a door and if you like it (and I suspect you might), either hire it out, or diy.

Tons of natural light, 8’ ceilings, brown leather furniture...... Gray? Taupe? Tan? Green? Help!

I suggest going a few steps darker than the cabinets. Your dark(er) counter top will be your inspiration. The grain in the wood is quite strong which can be used to your ADVANTAGE! If you water pop your wood, the grain will take on the stain a snick more...thereby giving you the DEPTH of colour you crave WITHOUT going so dark that you remove the light from the room (or the floors). I suggest a mid-tone stain with water popping. I would work a little into the 'green' range as your counter top 'reads' cool tones more than orange tones. Let the GRAIN take a strong stain (water popping) so that you can get all the contrast you desire WITHOUT having to go too dark. Make sure your refinisher offers some colour samples on your floor (must include a coat of finish to show the true depth of the colour...and if you water pop make sure the water popping is done with all the colour samples). Once your samples are down, you can take the time to figure out which stain works BEST in your lighting situation as well as with your counter top.