I've got 5 years of early retirement under my belt, and I feel like I've had the inverse experience of the OP. The first two years were an incredibly difficult transition period for me. Losing the status of being a higher up at a successful (small) company was extremely painful, especially because I didn't realize how much of a perk of the job it was. Like many others in this thread, I realized work provided a venue…
I wish there was a way to find a more balanced work culture (in the USA, other countries may have this already) to be able to continue solving interesting problems with interesting people, as you say, but also live life outside work.
I'd give anything to be able to work maybe 15-20 hours a week at half or a even a third of my salary. But employers will only take 60 hours a week of full-steam burnout, or zero.