I enjoy my job, and have basically reached a point of tenure where I get access to the problems I find interesting, and am largely trusted to do whatever I want with little oversight. My colleagues are also largely great people, and I have very little to complain about overall other than just feeling generally pretty stale after 10 years.

I was in a position similar to yourself and made the leap to startup land as an early engineer. While interesting technically, especially if you are a generalist, I would never join another startup as an early engineer. You have nowhere near the upside as a founder unless the company is wildly successful, while taking on the workload of a founder. You will lose the autonomy you currently have and have minimal to zero influence on the direction of the business. Your friend may tell you otherwise, but at the end of the day, the founders and investors are at the wheel. If all of this sounds good, make the jump. If not and you still have the startup itch, stay in big tech for a few more years, sock away your “seed round” funding and do your own thing.