Received FB offer but want to postpone start to keep working on my startup for a while. How to do it?

Situation: I’ve already left Uber to work full time on a startup idea. I was referred to the FB position a while ago before I made the decision to leave Uber and work on the startup, so I proceeded with the interviews and now just got an offer. Between starting the interview process and now, the startup has picked up some traction in terms of team/partnerships/validation and I would like to be able to work on it full time for a while and see it out before starting another big tech job, even though I think FB is awesome. How should I approach telling the hiring manager and recruiter about this situation? I’m thinking of just asking to make the start date 6-12 months from now so that they have a solid date. But my real preference would be to only get back to them in 2-3 months once I know better if the startup has a chance or not (postponing the decision instead of postponing the start date). Any advice appreciated here 🙏 Oh and TC was 160k at U, 2 yoe