Moving your cleats further back would give some more room to your heels and make your feet a bit more stable too. Moving your cleats toward the inner edge of your feet will move your feet outward and provide further room for your heels. Adding some pedal washers will add even more room - you should be safe to add up to 2 mm of washers between your pedals and cranks.

Nothing wrong with flat pedals though. With some practice you can learn to increase your cadence on them too.