I don't believe she's jeopardizing anything. I believe she's playing the same savvy game of "advance Nancy Pelosi's political career" that she's been playing for most of her adult life. If she's stepping down in 2021, she's doing it because it's part of her plan. The plan she's not going to jeopardize for Donald Trump.
That said, I suspect that her decision to pursue impeachment is motivated not by some savvy calculation that now is the time, but rather a groundswell of demand from her fellow Democrats that she can no longer resist without jeopardizing her plan.
"The die is cast" is, I think, an expression of resignation on her part. To me it suggests that she thinks it's probably not going to work, but they've gone too far to turn back now. She's spent the past few months trying very hard not to cross the Rubicon. But now that the crossing has been made, she recognizes that the only way out is forward and through, and make the best of it.