This just isn't true. Doflamingo is on a higher level than both Zoro and Luffy, and so is Fujitora. Sanji got embarrassed by Doffy, and Zoro was embarrassed by Fujitora, with the two antagonists being on similar levels.
There hasn't been a single person that has beat Sanji at Sanji's best that Zoro would have done much better in.
You went wrong at the second sentence. Luffy does train. For one, when asked why Luffy never gains weight with all the food he eats, Oda replied with it'd be impossible for Luffy to get out of shape with the all the exercise he does. When Luffy was separated from the Strawhats, the first thing he thought was "I better train while I search for them," his method of choice being walking on his arms the entire time so he would get stronger.
Sanji never training makes absolutely no sense. He's gotten stronger, developed new techniques(Diable Jambe), and his skills are still as sharp. The notion that Sanji doesn't train is complete fabrication. Like I said before to King Hashirama, there's no reason one can assume Sanji doesn't spend time training hard because that's simply what martial artists do and he wants to protect the crew just as much as the other two.
The world's strongest man's goal was to have a family. That alone shatters the notion that "I WANNA BE THE STRONGEST" means you're gonna be stronger than someone whose goal revolves around adventure or exploring. Correlation? Sure. But correlation doesn't equal causation. Nothing suggests the M3 aren't an exception to this correlation.
I'd love to see the equation you use to calculate "potential" since you seem to have a set, quantity of Sanji's.
Not really.
Don't get me started on this first mate bullshit again.
Muscle of the group? Hell, even Franky functions as "muscle" of the group seeing as he was among those tasked with destroying the factory. The group's main muscle is the M3 as a whole. There have been multiple times in the series where the cowards of the crew were scared, but rest assured when Sanji showed up, which is portrayal as "muscle."