Help re: job offer

I got a job offer last week. We negotiated terms by email and I gave a start date of 3 weeks from then. I requested a written offer and was told their lawyer is drafting it and would get it to me soon. Problem is Im still waiting for it a week later, and everything Im reading online says to never give notice to your current job until youve actually signed the written offer. I also read that it can take companies 1-2 weeks to get a written offer to you especially if it's a new position vs a replacement (which I didnt know and in hindsight I should have given a later start date) But I also need to give 2 weeks notice this Monday to make it to my start date. I did follow up mid week and was told theyre still working on it and to go ahead and submit an I9, which I did. What would you do?