Even though they were happy w/ just Touya and Fuyumi, this suggests Enji and Rei decided to try for more kids until they made the perfect one to show Touya he couldn't be the perfect child for his dad's goal. Also 'cause of Endeavour's growing fixation on making the perfect quirk

Despite having made Shouto, a perfect combination of their quirks, it's shown that Endeavour genuinely still cared for Touya and his safety since he basically begs him to stop using his quirk and develop a life outside of it as the damage it does to his body becomes more drastic.

With Touya, Endeavour tried to dissuade him from using his quirk at all, however it's very difficult to separate a child from the thing he believes gives him worth, especially in a hero centric society. The way they handled Touya ended up destroying him.

In contrast, Endeavour is very patient with Deku, teaching him to use his quirk in a less self destructive way, because Endeavour's seen first hand the damage an untrained quirk can do to its user. He sees Touya in Deku and doesn't want to repeat the same mistakes.

Shouto at any age: (⊙_☉)