<Spoilers follow>
Great read, but I can’t see how anyone would be optimistic about humankind after reading them: a group of humans who think we should be eradicated and invited aliens to do it, or at the least enslave us; collaborators when that eventually happens; and then as you mentioned, MAD, and then a series of bad, emotional decisions, all leading to near extinction.
I just can’t find any optimism in there. And it feels very realistic. I read the entire series during the pandemic, lockdowns, and the poor management of all of it, and the books resonated way too well.
To answer your question, I think it's worth reading them. I enjoyed them both. There are some smart ideas in there, the Dark Forrest Theory being probably the most famous, and one in particular that is overlooked: an interesting (fictional) explanation for dark matter. There are some good characters too. Also, they're better written (or maybe better translated?).
The story progresses faster into the future, with some characters remaining present throughout the books (cryonics FTW), and there are some ups and downs, so it's not all dark and gloomy and pessimistic. But overall, I maintain that the story is one of pessimism.