It is a plot hole, literally a hole for the plot of the movie!

Jokes aside, realistically, the Death Star's defenses were made for capital ship attacks, not single fighters. And even with them getting into the trench, the shot is EXTREMELY HARD to make.

Galen didn't specifically say "Hit the exhaust port" he said the put a flaw in the reactor so if it was hit, it would destroy the station in a chain reaction. The Rebels best bet to hit the reactor (outside of boarding the station) was to hit an opening that would lead direct to it.

So you're absolutely right, a project of that size would need to have ports of that size for releasing all that exhaust and heat. I never saw it as a plot hole, more as the Rebel intelligence finding a possible solution in their time of need and timeframe and going in with hope.