One more step along the road.....
If mixer direct to speakers shows the same problem but the DAW output meters show proper predictable activity on L+R, particularly when panning, then we're looking at the cables, the routing to or from mixer, or the way the mixer is set up.
It seems unlikely but if there's any manufacturer routing/mixing software with your audio interface, open it up and make sure that L+R outputs are panned as such and turned up.
I know my presonus software can mess up routing post-DAW.
Bt default the headphone outputs are disabled, which is always a nice head-scratcher! lol
After that I recommend taking one single cable from left output of interface to the mixer, and patching/panning on your mixer to try to make it audible via left, and right speaker separately.
Plug it into channel 7 then pan left, then right. It should move in your speakers.
Then Plug it into channel 8 and pan left, then right. Should get the same.
Mute the speakers in between - They'll probably BZZZZZZ with unplugged leads.
If you're able to do that and all seems to work, you've proven that your DAW left output is live and that the mixer routing and speaker wiring is fine.
Repeat but take one single cable from the interface
right output this time and do the same.
What you're proving here is the two paths and signals to the mixer, separately,
and the the two outputs from the mixer, separately.
Something there should shine a light.