I made a similar move a while ago, though I was not at the same level that you are and worked at startups before joining FAANG.
> Second, what are some of the surprises post-FANG that I would be in for if I wanted to start a company?
The biggest changes for me were psychological. I'm a competitive person by nature, and where I worked, most people were the same. It mattered a lot to me that I was keeping up with my coworkers, both in terms of salary and peer recognition. This was unconscious, and I never truly realized this until quitting.
As a result, it had become difficult for me to enjoy my free time. I would hate myself for doing nothing and wasting time. Let me give you an example. When I was younger, I loved playing videogames. After quitting I thought it's finally time to catch up with all those games I had missed. But after having worked in tech for so long I found myself unable to enjoy them on the same level I had before. Instead of taking my time appreciating the game and its art and discoveries, I felt pressure to complete it with 100% as quickly as possible, in the most efficient way possible. I believe that's what tech culture did to me. It made me into a robot trying to compete. That also spilled over into my relationships, where I would regularly cancel dates or leave early in order to optimize my time.
Getting away from this mindset took a long time and I'm still working on it. I am now slowly learning to enjoy the small things again, relax without worrying about wasting time, or keeping up with everyone else around me.