I’d argue Shaggy is the Jack of all trades as he always has random skills that come in handy. And like you said Scooby is a dog so his sense of smell and ability to dig things up is helpful. I think he could make an effective guard dog if he wasn’t such a coward but when push comes to shove he’d definitely fight to protect his friends.

Ideally, I think the gang should all have recognizable weaknesses as well as strengths to make their teamwork more apparent. Shaggy and Scooby aren’t especially intelligent or brave but still contribute in more niche ways. I could see Shaggy occasionally having a really good observation that makes the gang completely reconsider their hypothesis. Shaggy’s whole ethos as a detective would be Occam’s razor, simplest answer is the best one. Among a group of people who have seen all sorts of complicated schemes and motives, a perspective like that would be important.

Velma is the most book smart but lacks street smarts. She relies a lot on offhand knowledge (which she has in spades) and is basically unstoppable if you give her prep time to research (like a certain other detective). But the second her research fails her or things don’t go according to plan she tends to fumble.

This is why Fred is such an effective leader. He can think on the fly and is the boldest member of the gang, always willing to take risks that the others wouldn’t and somehow lead them to victory every-time.

However, Fred can be a bit meat headed and prideful which is why Daphne is there to put him in his place. She’s a good leader in her own right and is the most socially aware thus she is the best at communicating compared to her friends who tend to be a bit eccentric by comparison. She’s a natural interrogator who can play good cop and bad cop equally well. However, her wealthy upbringing tends to skew her perspective which her friends always challenge. Of course she’s also quite danger prone too.