I wouldn't go crazy chasing ventilation.

Ventilation doesn't help if the air can't physically support the necessary evaporation rate. It's physics. Just doesn't work in many conditions (hot/humid is common, also low speeds) - you'd need an air conditioner-dehumidifier with a jet fan pointed your head in order move enough dry air to pick up the moisture popping out of your head.

In fact a super well ventilated helmet with large vents will heat your head more if you have dark hair or a dark skull cap and you're not moving fast. It's like leaving your sunroof open on a sunny day - the sun will heat up what it can through the sunroof.

A good helmet helps keep sweat out of your eyes and ears. The Halo works wonders with this and makes up for a helmet's shortcomings. This frees you from going out and getting a helmet that is optimized for directing sweat back behind your ears.

I use the Halo skull cap too, but I found that the headband is marginally cooler when it's really hot and still sort of dry out. In humid weather they're sort of the same.

To help keep core temps down you can douse yourself with ice water, use Podium Ice (discontinued) or Podium Chill (half as effective as Ice) bottles. Chills will keep ice water cold for about an hour, meaning it'll be a bit warm at the hour mark. Podium Ice bottles will keep ice in your water for 2 hours. I've finished 90-100 minute races in 85 deg humid temps shivering because I was cold because I was dumping ice water on myself approaching the finish.