You're not wrong of course, but removing the tedious parts of a job can be really nice. I still work the same hours of course, but now I get to work on things that are more meaningful, more design and architecture oriented, and less about the grind of tedium. I'm sure there are plenty of exceptions out there, so I don't want to generalize, but I do suspect there are a number of people in a similar situation as me.

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.