How can we deal with this mental overload? There are only so many things that I can keep in my head.

Your job was to deliver Project X and you did it in half the time. What do you do with the other half? Your job was to deliver X. Now what? Extra vacation time? Just musing.

Honestly, I know several devs who do chores around the house or even play video games while AI does the bulk of the heavy lifting. If nobody cares or even realizes, does it matter? (To be clear, they all work remotely.)