IV/EV/Nature question.

I know, I know, you're all just rolling your eyes already that I just opened up this can of worms.

I've been reading about ev/iv and natures in order to get a better grasp on making a good raid pokemon.

From what I understand it really doesn't matter how a pokemon (going with an egg here) was born. It could have the worst nature etc. But with the items we have that can change natures, train evs, hyper train for ivs, I could just march 1 egg and make the most optimized pokemon.

Am I understanding this right? I feel like I'm missing something.