I got a job offer and verbally accepted it. However, since I still haven't gotten the contract, just the offer letter, which just includes very basic information and isn't even legally binding in my country - and even the letter was hugely delayed -, I continued interviewing in the meantime. The company (let's call it A) now wants to sign the contract on my first day, which unfortunately isn't unheard of in my country - although many companies want to sign the contract immediately after your accepting the offer of course.
So now I was told I got to the last, fifth or sixth, round of interviews at another company - B. The position is much better. I signaled them twice over the last weeks that I was at the final stages at another company and whereas I was massively interested in them, it would be great if we could speed the process up a bit. It's going very slowly, but it's a huge company, so it's probably quite common.
So now, I was told I was in the last round at B, but it's not known where and when this stage will happen.
At the same time I'm scheduled to start at A very soon.
What is the best course of action here given that B would be my clear preference but I don't want to stay without a job any longer since 1) I want to earn money again and 2) unemployment is driving me nuts (i.e. if B doesn't work out, I would definitely go for A). I can't delay starting at A.