I think that you're exaggerating how much pronounced emotion Spock shows in TOS. It's restricted to a handful of episodes, at most, and it's usually because Spock is under the influence of something, like the contagious madness in The Naked Time or the spores in This Side of Paradise. Now, I haven't seen this season of SNW yet, but if he's showing pronounced emotion practically every week, then that's way more often than in TOS, and I'm guessing that it's not because Spock is under the influence of anything because you're chalking it up to the immaturity of his character.
Also, I wouldn't say that TOS (or later) Spock is balanced. His whole character is defined as heavily suppressing his human side. That's why it leaks out only very rarely. It think that it's plausible that he was once more emotional and learned to control it, but more during his child and teen years. I don't buy that he learned to control it only in his 30s. Not even all humans take that long and Spock is only half human and very smart. I've always thought of him an overachiever, like 15 year olds who graduate high school early. In addition to being very smart for their age, they tend to be very mature for their age. The idea that Spock can't control his emotions even late into his 20s, despite his incredible intelligence, goes against what his character is all about to me. Besides, TOS Kirk never mentions how emotional Spock was when he first met him. He acts like it's the Spock that he's always know.