Is warlock 9 thief 3 a bad idea?

I'm doing my first tactician run and I'm using an archfey warlock. I wanted to lean into the charm aspect originally but it's kinda bad compared to fear, so I decided to just go for that and the invisibility that I get from my subclass. I figured that using those and other control spells for sneak attacks would be kinda fun. Also band of the mystic scoundrel and thief could be interesting. I'm mainly just concerned that it's too gimmicky or that pure warlock is better