I love smart MC in general, even if they're not the strongest.
Though I'm not sure if I can write them since I'm lazy and don't like to use my brain much.
What do you think is the easiest, most low-effort way to write an intelligent main character well?
Whenever your character is confronted by a difficult problem they can’t solve with their own knowledge, just get them to... google it.
A smart character doesn’t go into something blind, they will research before they go and ask someone else. What makes a character smart isn’t how much they know, it’s how they figure out a solution to what they do not know.
For example, your car broke down? Do you bring it to a mechanic immediately or research things that might have caused it?
Is it simply a battery problem? Something easy to fix? If it is, then your character does it on their own. If they acknowledge the task is something beyond their expertise, then they seek out assistance from someone they can rely on.
Smart characters do not only rely on just themselves. They are able to acknowledge what they can and cannot do and are able to recognize that there is a limit to their ability.