Great idea, I am still doing it this way.
In my opinion, there are some more points:
- shadow convoys till it gets night, then attack (send reports to BdU regularly)
- no harbour attacks
- under no circumstances attack any destroyers or corvettes; one exception: self-defense; of course large battleships are allowed
- just use deck gun to sink damaged and not-sinking torpedoed ships and not to sink ships themselves; exceptions may be very small, but plausible ships (no small fishing boats)
- confirm first the status of your aim (i.e. if it is an enemy)