By default, as of 8.1.1, the recipe for shadow metal fortress armor is quite cheap compared to how strong it is, would making it require void metal armor be good enough to balance it?

Did you mean requiring void fortress armor? It already uses void metal armor as its base.

As for my own opinion, both void fortress and shadow fortress armors should either be toned down in power or toned up in cost.
-Remove the vis cost from void fortress armor or make it require void robes instead of the normal armor. This way, void robes still have a purpose, either in being an alternative or in being a requirement.
-Lower the armor values of both fortress armors. They have incredibly ridiculous physical resistance. Making void fortress armor require robes while still having thaumium fortress armor's defense would make it a great way to combine the two, and then having shadow fortress armor have a stronger defense on top of it as an upgrade.
-Perhaps, in order to make up for the loss of armor points, including some extra utility on shadow fortress armor. For instance, the helmet could the effects of all three faceplates at once, or offer new faceplates altogether. Could buff melee or magic damage, or add void resistance, or sprout eldritch wings that allow you to fly at great speeds.
-Make shadow metal itself much more endgame, requiring eldritch research and an involved recipe to ascend void metal.
-Make shadow fortress armor's recipe justify its power. Adding a primordial pearl, a nether star, or something else of that nature to each recipe may be a good idea. Making void fortress armor required instead of regular void armor could also help.
-Lower fortress blade damage, but offer the ability to enchant. This is more for convenience than balance, as it allows for better adapting to environments that have stronger enchanting abilities than normal.