Yeah, you're hitting the soft cap on Illusion's effectiveness, not a perk problem. The spell itself doesn't scale with enemy level past a certain point - Fury and Mayhem have a base magnitude that doesn't care how much your skill is, so at 52 with leveled enemies in the 40s-50s range, they're just naturally resisting harder. It's a vanilla design thing that sucks.
The pickpocket part is separate though. Are you using the perk that lets you pickpocket without being detected even if you're caught mid-animation? That one's essential for this playstyle. Also, Pickpocket skill itself affects success rate way more than people think - if it's lagging behind your Illusion, you're gonna get caught constantly. What's your Pickpocket level at?
Real talk: if you wanna keep the illusion assassin thing viable past 50, you either need to swap to paralysis (scales better with enemy level, actually stops them), go full destruction mage, or accept that Fury/Mayhem is a mid-game tool and pivot to straight stealth + backstabs. The perk that increases backstab damage is still absurd at any level. Or mod it - Ordinator and Apocalypse both make Illusion actually scale properly if you're on PC.
What difficulty are you playing on? That affects enemy resistances too.