Congrats! But be very careful about the start up you're working for. Make sure they have enough cash to survive the next 4-5 years and have a steady revenue stream.
If this is your criterion for working at a startup, then you are too conservative to work at one. Every job carries risk, and looking to eradicate it is pointless.
If things tank in a year or two, OP will just be the target of a recruiter feeding frenzy, and they'll come out more qualified than they went in.
Quitting immediately would look bad in comparison, and staying but exuding pessimism and paranoia would not be adaptive.