Two clues: Both several sets of tires, AND I think caller stated a 1997 Subaru with 46,000 miles. That’s not a lot of miles for 13-14 years, which makes the tire wearing even more dramatic. (Could there be some wheel / axle / differential problem that is slightly dragging the tires and wearing them out? Would problem be worse not if temperature is warm or hot, nor if drive is uphill or downhill … but if drive involves lots of sharp turns, and thus more dragging of the tires?)
Maybe problem is similar to something I had with my car. I had a low-mileage 1996 subaru legacy, with infrequent / occasional burning smell – it usually only happened after a long drive. My problem was never solved. But mechanic suspected it was connected to car not being driven sometimes for days (I work from home). Thought was an old car sits for days; rust or other sticking problem forms on some part of wheel or axle or brake caliper; then a long drive, extra friction eventually generates excessive heat.
I, too, only smelled the burning smell when I got out of the car – smell was near one of the front wheels, but that’s also near the axles, and cv joints and engine, and …etc