What about a factory FE5 strut? I'm running the Bonneville GXP suspension on my 04 LeSabre, which uses FE5 springs, and bought new OEM FE5 struts to go with them. Even compared to the Monroes, it has to have a lot more dampening, due to the higher rate of the FE5 springs.
I started out with an FE1 suspension, and followed a member here who swapped FE2 for FE5.
What if you were to combine a stiffer lowering spring with an FE5 strut setup? I'm not sure of the relative stiffness of your lowering springs to a stock height FE5, but something like the FE5 strut might compensate for the increased spring rate. But then, the drawback is that the FE5 came from an 8th gen H body. Not sure it would fit in an earlier application without altering your geometry.
Also, I know some of the Monroes, the Sensa-trac'*, the dampening effect changes where the piston is. If you go to lower a car on Sensa tracs, the strut isn't working within its normal range, so it will completely mess up the ride. The whole idea is that it moves very little in the normal height range, while it stiffens up at the extremes. If you lower it, the whole setup will be inconsistent. You want a strut that has a constant dampening factor, not something variable like the Monroes.
2004 Buick LeSabre Custom FE5 GXP suspension conversion. 170Kmi
2001 Silverado 4WD 5.3L, 33" BFG ATs, Z71/Z82 pkg, 147kmi.