>Who: Series B startup with ~$100 million in funding, me open-source developer

That $100 million isnt in your pocket. This isn't 100% reliable source of income. It might seem like alot but it's not established yet.

>In a meeting they explain how they were interested in using my open-source project and want to hire me so i could carry out a similar work for them

Sweet, i assume they want to use your code. Might try to be tricky and get you to sign your code away. Then fire you and keep the code base.

>Then they go on and explain i’d have to do 4 more meetings 1-2 hours each, including a leetcode-style tech interview

Kind of a red flag in my books. If they are coming to you, then it's the reverse. Like they should be offering you equity or something. I dunno. They need to win you over and bureaucracy isn't how to do that.