Early works set in the Noon Universe[0] by brothers Strugatsky were pretty optimistic in spirit.

0: https://en.wikipedia.org/wiki/Noon_Universe

I also wanted to mention the Noon Universe, but I have to point out that two of the most famous books of the series(Трудно быть богом - Hard to Be a God[0]; Обитаемый остров - The Inhabited Island[1]), which are also my favorite books of the series, are also sort of set in a dystopian world. They had to sneak some dystopian worlds into the utopian universe to make it more interesting.

[0] https://en.wikipedia.org/wiki/Hard_to_Be_a_God

[1] https://en.wikipedia.org/wiki/Prisoners_of_Power

I'd say that everything post Hard to Be a God had either a streak of disillusionment or something like anxiety over, idk, incomprehensability of the universe and progress, but early works were pretty uplifitng.

"Noon 22nd Century" was very optimistic and inspiring, I really liked it as a kid and dreamt of becoming like the scientists in these stories. Even something like "Far Rainbow" had a grim plot but an uplifting sentiment regarding the nature of human spirit so to say? The tragedy happened but people faced it with strength and honor, so to speak.