I think the biggest problem is forced mechanics that try to "brute-force" a ship into a role, rather than letting their strengths play out naturally.
A capital without range advantage can't do anything against a much faster DE. As a DE I'd just attack from exactly max range retreating just a bit when I need to vent. 2 DEs could easily tag-team any capital that doesn't one-shot them with TLs.
Why is that an issue? This a FLEET game. The capital will rarely be alone. And even more reason to have your own smaller, faster ships in a fleet.
And 2 DE's might not have enough DPS to tag-team. Remember, IF range depended on mount size (a large autocannon will have a bigger range than a small one), the capital will STILL have a range advantage on the account of more bigger mounts. It only won't be as overwhelming as it is now.
If you increase officer limits and change fleet limits to FP, rather than ship count, mroe diverse fleets would result.
Lastly, you have to think WHY smaller, lighter ships exist to begin with. From costs (abstracted, defense fleet upkeeps do not exist. If each fleet cost you upkeep depending on composition, players would act differently), the abillity to cover area (patrols) and interception/speed (if you persue a fleeing enemy, only smaller, faster ships get to flank)