I think the really good part was "should I go back and end all possibilities to avoid future conflict".
But all the "light and dark should fuse together" element didn't fit too well with the previous series. Chaos didn't just want light, it wanted to destroy everything.
Yes and no.
No because it went against the ethos of the earlier arcs.
Yes because Stars was kind of a deconstruction of Sailor Moon/magical girl tropes in the first place, before ultimately reaffirming them.
You could say it was the series growing up, but doing so without rejecting what it had been before.
Instead of the magical power of love miraculously fixes everything, it fixes everything in the immediate present but we are told things will become awful later.
Cosmos is blackpilled, her pov is that all the suffering she has seen could have been avoided if she chose to destroy the cauldron, which would have meant no new life would occur. Her pov was it is cruel for life to endure if it means more suffering.
Usagi's pov was that suffering was a price worth paying if it meant life could happen, that love and happiness was a possibility. Which is poignantly also the case for marriage, you know even if everything goes well it will hurt real bad when one of you eventually dies. But people get married anyway, and in usagi's case she literally has a new life growing in her when she marries
Basically Cosmos was a glass half empty vs usagi's half full.
Cosmos rediscovered who she was by reintegration her younger self into who she was. She knows the future is bleak, but life will endure and that is worth the suffering along the way.