The AI uses my designs? I thought i saw that happening! lol
The answer is that i usually do not bother upgrading except the ship which i am currently building, and there is usually only one type of powerful ship i build - which is a high defensive escort. I then fill the logistics with rubbish ships i traded from minor races.
If and when i have to take a fleet into battle, and it looks difficult,,, only then i will update ... and the only important change i make to the crappy ships i got from the AI is to make them 'capital'.
One defensively powerful Escort combined with whatever crap you like that are capitals is a pretty invincible fleet,, your jaw will drop when you see what it can do! lol
If i want a super powerful fleet,,, i will design one tech ship, combined with offensive tiny ships set to capital, and one defensively powerful escort with the largest hull i have, tailored to the primary weapons of the enemy. All set up with my latest weaponry,,, but mostly i don't need to bother doing this.