It posits a world where the Soviets won the race to the moon and reaped enough cultural and economic benefits to stay together, and the space race never ended. Various political events play out differently, and somewhat different things happen culturally and very different things happen technologically. It does a season per decade, and from the thumbnail it's no secret that humans make it to Mars.
Imagine if you began with "The Right Stuff" and started down a path towards Star Trek a decade at a time. It's far from perfect, but it's a fun ride and has a lot of "smart people solve hard problems" scenes.
It's an alternate-history telling of the space race. In its version the USSR makes it to the moon first, so the US doubles-down on their space program and continues beyond Apollo.
for all mankind is like watching a badly written soap opera at 2x speed but twice per season there is some kind of huge space catastrophe and a bunch of astronauts die
I was hooked for season 1 then very quickly found myself wondering wtf was going on in s2 only to be completely off put by s3. It really is just a soap opera.
Most of Season 1 is basically if somebody walked onto the set of The Right Stuff and replaced everybody's scripts. It takes into Season 2 for it to really get sci-fi-ish at all, and it's only S4 where we're past "The Martian" in tech level. If you are intrigued and just want it to get a move on, yes, it'll get "better," but change happens incrementally, and a lot of the drama is earthbound or at least human scale, and some plotlines are better than others.
It's not really a sci-fi show. It does take place entirely in an alternate reality though, and features newer technological advances, but those are often in the back seat to the drama that males the show so entertaining.
It's mostly sold as alternate history for the space race, but it's first and foremost a drama.
Drama about people that happen to go to space in an alternate world.
Drama about people having sex with >! their dead children's best friend who they also raised once their parents went to space and died, the repercussions which amazingly happened for multiple seasons, it's kind of weird. !<
The driving force of the show is not the alternate history.
The entire first season is more alt-history than sci-fi, and that can take some of the wind out of your sails. The ratio changes slowly over time, but they're trying to at least plausibly start with our world, circa 1965, as a jumping off point to fork the timeline (I think the conceit is some key soviet scientist and administrator who died in real life survives here and quietly keeps them on track).