Should I join a promising early stage startup or a FAANGMULA company?

I need to convert this into a course, but I still recommend this: [Masterclass] How To Choose A Good Company And Team As A Software Engineer

In short, it depends on your priorities. While a startup will have more velocity, scope, and learning, Big Tech brings compensation, stability, and credibility. Since you're a senior engineer, it can easily go both ways as that's when you have the most optionality (in contrast, I recommend junior engineers start off at a Big Tech company if they can).

Check this out too: "What's the difference between Big Tech and startups in terms of learning and growth?"

I like this advice from David Pan, VP of Eng: Figuring Out The Best Company To Join As A Software Engineer. Basically it boils down to "Follow The Talent You Know"

Should I optimize for learning with an AI legend

This could work, but the main thing I'd want to know is how much interaction you'd have with the legend. Be careful of famous people doing startups, but not actually being very involved in them.

Do you have a large brand-name on your resume already, or some credibility otherwise? Sounds like you do as a past founder and open source contributor. If you don't fee like you have the logos on your resume, I'd lean toward taking the Big Tech role.

There are tons of AI companies in the post PMF era that are doing really interesting stuff. I would suggest looking into unicorns or late stage high growth AI startups. e.g. perplexity, elevenlabs.io