I'm in a similar boat currently. I've been in the same startup for almost 6 years now, pretty much since day one of its creation. I've been with the founders through every step of the way and my closest circle of friends is among the employees.

But at this point it feels like the product is going nowhere fast. I'm worried about my career (i don't have much else on my CV) but being the very first hire I'm afraid quitting would kill any confidence remaining in the company and trigger an exodus.

If things at your startup have been run equitably, then after 6 years you should have some equity that's all yours.

That makes it fine to quit. You'll stop getting salary, but that equity is your stake in the future as things unfold.

If you don't have any equity after 6 years then it's time to bail anyway.