Might contain mild spoilers, but i'll try to make it as vague as possible for anyone who haven't seen the anime.
First, let me adress the "prequel" issue.
I don't think this shoul be called pequel. Yes, it's a story that's hapenning 6000 years before the anime story (6000 sound really weird, given certain ppl are already around), but it's being told by char you know from the main anime (and is not technicaly fully around at the time when the events begin). Also in the end there are characters you get to know in the anime and the list pinpoints the story is being told after the events of the first season. So technically it's a sequel story of events that happened in the past.
That being said i really think it's best viewed after the anime itself (since you can reflect on the state of the world and some characters and realize how they changed).
The story itelf was definitelly more interesting than the 1st season (even though it was a classic story). The visuals were much more bearable (the season 1 was sooo much oversaturated i had to lower the saturation by 30% and brightness by 10% to not have lasting color burn damage). It also have way less (aka almost none) cringe "fan service" situations.
Apart from that the music was ok, as well as the sound design.