Would just lowering the cost of the minefield be enough? Imo the area denial part of s-mines a good idea, but cost prevents larger fields. If they were cheaper they could be used as intended to delay enemy flanks and movement, forcing the enemy to dedicate manpower and time to sweeping.
Dont build whole 4 sectors. If you want them in straight line build one sector and then proceed to another along the way. You can make a really long line that way spending 45, 60 at most 75 munitions depending on length. Usually though you just want them placed in narrow passage to block flanks. Then you need only one sector. There is rarely more than 2 passages on one flank so it means you can cover whole flank for 30 muni which is the price of one standard mine.
One and only thing I would like changed about them is disabling cost check so that I could build one sector with only 15 muni in bank.