Problem solved!

So it turns out that the lowest bolt on the safe door had some grease that had dripped on to it from the locking mechanism above. Over time, that grease had smeared out across the bolt, attracted some dirt/dust, and was now all gummy across the surface. I only noticed the issue when I was running my fingers through the shadowy recesses of the door frame and felt it - it appeared perfectly clean to the eye.

A quick wipedown of the bolt with a degreaser solved the problem instantly. That gumminess was causing all the drag when operating the handle. Since I had the whole mechanism exposed, I also cleaned the rest of the sliding surfaces in the safe and regreased them. The door action feels slicker than snot - like it is brand new. Feels good to solve a problem

Hope this helps someone in the future.