Yes, a certain build can help but it’s ultimately about numbers - numbers always win no matter how rubbish the ships are in my experience (two full campaigns and many aborted ones).
I think that Stellaris just brought home to the space genre a well established historical fact which is best summarised by Horatio Nelson’s dictum: “Only numbers annihilate”. The only mechanism that has historically kept in check mass is deception and friction. Not surprisingly, both of the latter concepts loom large in present day thinking on strategy and tactics.
Deception, in the sense of misleading an opponent with superior numbers or firepower to deploy these assets in an ineffective manner, is not really possible in Stellaris against the AI. The AI knows where your fleets are all the time. There’s no real way to force an opponent to become overextended because redeploying fleets is very simple. Surprise in Stellaris is most often achieved by means of the assymetric FTL system: something which has been deprecated.
Friction doesn’t exist at all. Every ship gets right what is supposed to do every time. No ship component turns ever to be faulty. There’s no misjumps or any other hazards affecting the strength of your fleets. It does not seem to ever be anything like a “critical hit” in the combat system.
I think the redesign slated for Stellaris 2.0 will try to address some of these shortcomings in a staged manner.
But I remember in Moo2 my smaller fleet would consistently out fight enemy hordes. On the harder difficulties, it was the only way to survive let alone win. I find that fun (and moo2 had a lot of personality - for example getting to Orion first was a big deal, you felt the loss of a hero, and I would find ways to guarantee their safety. In stellaris I never put a “lifeboat” option on my fleets. If Admiral X dies, Admiral Y will be fine).
That was you, looking for the synergies between ship components that the AI hadn’t been told existed :-) That’s a puzzle aspect in ship design and fleet composition which I also find very rewarding, and that I don’t feel like I get to experience very directly in Stellaris.