A known quantity is offering you a 20% stake, that's huge. However, they aren't offering you control; those seem like the terms. Previous success is correlated with future success - so, the opportunity is sounds great. If you can't live with that, don't do it.

The terms are great, but it doesn't sound like what you want. Realistically, a board seat is a huge ask, and 3 people from the company on the board, isn't typical, especially at this stage.