Ship design in a 4x really only becomes relevant if the A.I. is willing to adapt to your designs. I remember numerous times in SOTS 1 when that happened, e.g. I had a missile heavy fleet that rolled over enemy fleets until one day they turned up with a load of PD ships. Same with armour and armour piercing ballistics, or lasers and reflective coating.
Stellaris forces you to adapt to the A.I.'s fleets but I’ve yet to see an example where the A.I. has adapted to yours. If/when that happens, Stellaris will top SOTS as my favourite 4x game.