I would not prune off any of the branches until after your spring repot.
Your heaviest branch, largest in diameter, leans to the left. This would in my mind be the main focal point of the tree. Explore the roots, if the roots allow you to move this tree to the center of the pot, or slightly right of center, will allow you to use that heavy left branch. The roots will determine where in the pot you can put the trunk. If I could, I would use that largest diameter branch either as the "main trunk" or at least as part of the clump if you go with a clump style. But until you see the roots, you don't know what you can do with it.
Nice looking cotoneaster. I'd be happy to have it on my bench, looks like a fun one to figure out.