> First, what is the environment like for exiting high-level former-FANG employees?
For most employers, it will be a positive. There may be a handful of very small startups that see it as a negative, but the reality is a very small startup is so different from a giant tech firm that the hiring team is probably correct in assuming you've never had to do something incredibly arduous without enormous support (I once had to spend 6 months reverse engineering a video output because our customer refused to tell us what it was - I ended up finding a clue on a Russian blog and I translated it with Google - this is the sort of shit you will do at an actual hard-tech underfunded startup. And if you care, it turned out to be a proprietary video signal invented in the 80s that had long since been abandoned by almost everyone so I then had to build a fucking circuit to decode it).
> Second, what are some of the surprises post-FANG that I would be in for if I wanted to start a company?
Your reputation will help you with investors, but it will not help you with reality. Starting a company is much harder than you think it is - no matter how hard you think it is; it's harder. That thing I said above about reverse engineering a proprietary video signal because our customer was impossible? That was one of the most enjoyable parts of my job.
If you want to start a company, and you're able to do so (don't put your family savings into jeopardy over this), go for it! Just realize it's not going to be anything like a giant tech firm, where you don't realize they actually do 99% of the job for you. It will be far more rewarding. It will be far more stressful. There is an extraordinarily high chance of failure. You will likely make friends with new peers, and likely ruin some relationships in the process.
The best advice I can give anyone that wants to start a company: ask yourself why do you want to start a company? If the reason is anything other than "there is this huge problem that needs to be solved and I know a way to do it!" then you are chasing a bandwagon of the glory that comes from being a successful technology entrepreneur, and aren't actually building a company with any value behind it.