Hmm, I gave this whole subject only passing thought when developing my longest played 5E PC.
He was a Wizard, so yes I started him with a decent Int. And we played with Feats, but I chose those feats not primarily for the mechanical benefits, but to fit him in a roleplay sense. He was a bookish nerd, who was pedantic and examined things to the nth degree, hence 1st Keen Mind, and then Observant fitted him thematically. The fact that both grant a +1 to Int was a nice little bonus, but was not the deciding factor.
But whether he got "+loads" with his spells, not really a worry. "+something" is all I cared about to ensure he was useful. But usefulness lagged way behind the personality and interaction at the table in terms of importance.
So it's a begrudging option #3 for me, as I did put the 3rd ASI/feat choice into +2 Int which took it to 20. But not for mechanical ooomph, just because he spent ages in libraries building his knowledge and it fitted his character.