I'm the OP, and I do wonder if I would be too old for a FAANG, with the fault being on me. Building a company as an entrepreneur at age 45 (my age) feels very reasonable, and I've seen research that suggests that my age is perhaps even a sweet spot for building a company. In contrast, I think I would find it difficult to work as an employee at a FAANG company, since it's very different than everything I have done so far in life, and most other employees would be younger than me (informal observation based on having lunch at Amazon, Microsoft, Google and Facebook regularly).
More importantly, joining a FAANG company would mean completely giving up on my longterm goal to massively fund Sage development, and that would be very hard for me to do.
While most other employees at any tech company would be younger than you, is having lunch at AmaFaceGoogSoft really so different from having lunch in a university campus cafeteria?
I am not trying to change your decision, BTW. Your long-term goal is an admirable one and I think achievable, whether CoCalc remains a private company, goes public, or is acquired.
More importantly, joining a FAANG company would mean completely giving up on my longterm goal to massively fund Sage development, and that would be very hard for me to do.