The Venture-class Corvette is, in WC1 terms, a compromise between a heavy bomber and a destroyer with the firepower of the bomber with the lack of maneuverability of a destroyer.

The only reason it's made really is because it is THE smallest capital ship you can send out that's a capital ship in WC1 terms, yet it's small and wimpy enough to be cheaply made and thus can be placed where you want some sort of Confederation presence without assigning one of your more valuable and useful capships to there.
The Diligent was converted Drayman-class transport, IIRC. It's just a transport which had some important plans which had to be recaptured or destroyed.
And the Exeter in WC1 is a destroyer - it's not meant to go up against fighters, which can chop it to pieces because it can't defend itself adequately. It's meant to go up against other capships - other destroyers, cruisers, etc.
And as for WC2...
Your Gilgamesh destroyer is an evolution of the basic design philosophy that guided the Exeter-class series' construction: fast for a capship, meant to go after other capships. Depends on fighter escorts from someone else.
Waterloo - different role from the Gilgamesh, meant to be a Heavy Cruiser which could defend itself with its own fighters, yet also have serious firepower to direct against other capships... having four AMGs to the 2 that the Gilgamesh carries. Yet far, far more expensive than the Gilgamesh, for the extra resource expenditures building a ship that can fight with both fighters and guns takes. The Gilgamesh is cheaper, which means you can put out more of them to cover more systems.