Being bulky or fat does not mean you're slow, look at Jozu and Lucky Roux, both Yonko heavy hitters.
Needing brute force is all fine and good but I don't think the suit adds brute force primarily, it adds brute force sure but thats kinda secondary to everything else. so having an opponent that needs brute force like jack or queen to showcase a powerup that is primarily mobility, durability and versatility based is just a worse showcase. Imagine if katakuri had been the showcase for boundman and Jack would have been the showcase for snakeman. Snakeman wouldn't look as great dealing less damage to jack and boundman wouldn't look as hot not being able to hit katakuri.
There's no hidden rule that Sanji can't get big or bulky characters (see Wadatsumi). Sanji is not Luffy in that he has different forms to handle different opponents. Why should he be limited to "fast" characters but tap out at bulky guys? How are Queen and Jack a poor showcase for mobility, durability and versatility? It's like you're throwing buzzwords around.
He had a sword through his head a few chapters ago and Kizaru turned him into a swiss cheese in MF. As long as his regeneration works he seems unkillable. Which is why he can stall anyone. Even people way above his pay grade. The problems for him start when is regen runs out.