Not really, mods that have all the functions they are made for in one mod are actually extremely convenient to maintain because you don't have to coordinate compatibility between two different mods from two different modders or check both for issues when there is a problem, it is all in one convenient place. When something doesn't work, you don't throw the whole mod away, you fix the problem, which is a nightmare between two different modders, especially when the modding community has already refuses to maintain compatibility.
Illusion has everything to do with the FPS issues, those are all because the game uses RAM almost exclusively. The more characters being rendered at the same time in high poly, the more RAM it takes to keep the frames high (it takes up to 32GB to run a save with the full character limit without forcing the high poly models, it takes up to or more than 64GB of RAM to run a save with the full character limit that is forcing the high poly models, and both of those are without the mods that give more slots for girls, which adds even more RAM needed). You said it yourself, the game wasn't designed for it, which is Illusion's fault. If the game isn't designed for something, that is simply not fixable, mods inherently have the potential for issues and can only go so far beyond the capabilities of the game.