I've been trying to wrap my head around this for a while - I've got a circle sliding down an inclined plane, in an entirely non-elastic reaction, and I can't figure out how to slow it down accordingly. I'm not simulating angular motion, as in the circle doesn't explicitly rotate, it just slides down. Would anybody be able to help me understand the correct force to apply for simple sliding friction on the circle? All the articles / explanations I've found either explain entirely static friction or between 2 moving bodies.

Two bodies moving across each other is the same thing as one body moving across a stationary object, it just depends on your reference frame. What you're looking for is Kinetic Friction, which is proportional to the normal force between the two objects (i.e. how hard one object pushes on the other and vice versa), and always directly opposes the direction of motion.