Did exactly that move (FAANG to founding a startup back to the same FAANG, right as everthing was melting down with COVID). Some advice based on it.
Leave on a high note. My last year was kinda miserable for me because I didn't really want to be there, but I was determined to deliver some value for my department and complete some projects successfully. Paid off because when I came back, my skip lead was now the senior VP in charge of signing off on offers, and he remembered my name and was determined to get me back into the company. Also had a bunch of other people I'd helped out in that last year pulling for me to come back.
Learn all you can while you're at the FAANG - there are many opportunities there you won't get at your own startup (and vice versa). I think I did that well on the technical side, but I wish I'd accepted management opportunities when they were offered to me. Also do your best to understand the organizational politics and incentives acting on big companies; they often set the agenda for smaller niches in the tech world.
Make a lot of contacts, add them all to your LinkedIn, and then maintain them once you leave. I did the first two well, the third not so well. People move on while you're gone, so this maintains a network that may be at other up-and-coming startups.
I stayed away too long (6 years). If the startup isn't working out after 2-3 years, call it and jump back into whatever the hot company is of the moment. Many FAANGs will let you re-enter without re-interviewing, at the same level you left at, if you do it within about 2 years.
You might be surprised how much you retain going back into a big company (at least at Google and Apple, and I suspect the other FAANGs). I kept my LDAP, recurring calendar invites (!), pending code reviews (!!), doc ownership, bug ownership, code submission history, vacation seniority, language approval powers, familiarity with the bug & code review systems and most of the internal tech stack, company credit card (I returned the physical card when I left, but when I returned I had the same account and same transaction history), equity award account, and a lot of relationships with coworkers who were now at the VP/director level. I lost logs access, code ownership, and unvested equity from my first time around. It was sort of like stepping into a time capsule, where I come back and have code reviews and calendar invites waiting for me from teammates that long since left the company.