I don’t know what’s next after the PhD. It could be another FAANG job, founding a startup, or really anything. A PhD definitely does not help get me there. It’s just a personal goal
A whole lot of comp sci research doesn't need a lot of resources, so you can do that anywhere, it's just a question of time.
Some research needs lots of computer time and/or lots of real world data. You likely have access to more advanced computers/more computer time at a FAANG, at least in theory (getting to use it is something else) as well as real world data.
What does a credential do for you here that FAANG on your resume doesn't?
1. You have access to paywalled papers. The majority of leading edge research is published in high subscription fee journals.
2. You gain a great deal of knowledge and build international network of contacts by attending conferences and collaborating with others. Sabbaticals are another form of leverage.
The latter though you really have to figure out if that’s worth doing unpaid labor for. Seems like you could leverage your existing industry position to network with professors better then you could as a phd student. You could reach out to professors of interest and offer to mentor their phd student in the ways of industry or help them find jobs and use that as leverage to get access to folks you are interested in