The damage still sounds a bit low to me for a 20s reload rate. So how about 12 million with a 10 or 15 second reload time. That should still be slow enough to be "reactable" against, or even "brute forceable" against, but still have the DPS to be threatening against a large cluster of higher mark starships.
Also, while losing a starship means grave economic damage, merely repairing a starship isn't all that painful economically. That's why either a strong burst damage or a high DPS is preferred in my eyes, without at least one of those, it won't do it's job of forcing that choice of avoiding starships or major economic losses.