I started at my second 'FAANG' company when I was 48, two years ago, and it's been great. The previous 'FAANG' company was less great, but still fairly ok.
In between and before those, I have worked at two startups and three other in-between (in size) organizations, as well as two non-tech giants, over the course of about 27 years now. Kind of takes my breath away seeing that last number!
As others here have noted, no matter where you work, one of the most important things is your manager. I've had good managers and bad managers, and my less happy time at the previous 'FAANG' was largely due to a bad manager.
Having said that, the department/group you're in is also important. I had a great manager at one of the large (but not giant) companies but the department had big problems. It was a relatively comfortable but in the end unproductive couple of years. (And also why I decided to move on relatively quickly.)
Re: skills to brush up on: honestly, I suggest just start interviewing heavily and use that as a template to figure out what you need to learn.
The big tech companies have the luxury of having tons of talented, qualified people trying to work there, and so structure the interviewing process to be biased toward turning away qualified people rather than accepting unqualified people.
That means that the interview process can be pretty strenuous.
Re: trying for management: I was a manager briefly, early on in my career, and decided that I was never going to do that again. I was relatively good at it, but I didn't like it, so all of my roles have been technical, though of course a lot of the best things I've accomplished involved large quantities of 'soft', non-technical people work.
Re: fed up with uncertainty. Yup, I totally get that, and it's the main reason I haven't done more with startups. I value my personal/family time too much, and always have.
Feel free to contact me directly: diederich@gmail.com
Best of luck.