Illusion's actually viable, but you're hitting the hard wall where the school's design breaks down - the perks don't scale with enemy resistances, and that's the real problem. You're not wasting perks if you commit to the right path though.
The key is the perk that increases the magnitude of your illusion spells (should be somewhere in the early/mid tree). That's your lifeline. Stack that with the one that lets spells affect higher-level targets, and suddenly Fury / Frenzy actually land on stuff that matters. The magnitude boost especially matters because even when an enemy resists the spell, a higher magnitude can push through the threshold. It's not elegant, but it works.
Beyond that, you want the perk for dual-casting (boosts magnitude further) and the one that silences casting noise, mostly because you're gonna be in range of your targets and standing there channeling spells is a great way to die. Quiet Casting is almost mandatory for illusion builds imo.
The real thing though? Don't lean on crowd control alone past mid-game. Illusion shines when you layer it - use Muffle to stay undetected, hit them with Fury while they're confused, then backstab or nuke with a secondary school. Pure illusion-only builds do struggle. Pairing it with Destruction or Conjuration makes the whole thing click because you've got fallback damage when the spell misses.
The Necromage thing is a crutch. You can absolutely do straight illusion without it, just don't expect to solo a dragon with Calm spells lol.
