I did some brainstorming and had an idea that I quite like, however it would have to be implemented with an addon.
One of the main reasons why the small/medium ships are hardly built is that the tech10 ships can do everything better, at the cost of more fuel for moving but with the benefit of greater safety (from ambush and minehits).
So my proposal to enforce the utilisation of medium ships as minesweepers would be to double the mineral cost and increase tenfold the cost in mcs for beams that are put on ships with a techlevel higher than eight.
Technically, the addon would just have to check the techlevel of the hull of new builds, and check if the starbase holds nine times the mc cost and the same amount of minerals as the beams cost. If the stuff is on the base, the ship gets built, if not, no build.
Maybe the increase in cost should be only triggered, when the techlevel of the beams is above 4 or so.
edit: of course the 9 times cash and the minerals are removed from the base.
Wirklich reich ist, wer mehr | Truly rich is, who holds
Träume in seiner Seele hat, | more dreams in his soul
als die Realität zerstören kann.| than reality can destroy.