Facing with the same problem (not exactly: my keyboard have a few more keys than the Corne), it always felt more disruptive to me to interrupt the flow with layer switching than to stretch the pinkies to the outer column (or middle fingers to an extra row) for uncommon characters.
So I used Bépo (and now use Optimot) with very little adptations.
It implies outer column stuff must move either to thumb clusters (for the most used keys) or to layers, to give priority to letters.