Thanks for all the help so far.

I've dry fired the camera 50+ times. It didn't affect the light metering, but the film advance smoothed out even more.

Changing the ISO back and forth many times didn't change anything.

I have a test roll in it now. In auto mode it seems to be close. I figured out my manual settings with a separate light meter, and listened for how long shutter click was. I then took the same photo on Auto and the shutter was either the same or very close. So I think Auto mode works (I'll find out for sure when I develop the roll).

The meter reading in manual mode is more buggered than I originally though. While it does scale with changes in ISO and aperture, it doesn't change based on how much light it is getting (mostly). If the lens cap is on, it reports nothing, with the cap off it reports the same light reading no matter how bright or dark the scene/subject I point it at are. The secondary mirror behind the main mirror is definitely hanging down in what appears to be the proper position.