I think we have one obvious reason why season 1 was so solid: Bryan Fuller. He came to Trek with fresh ideas and thoughts about how to use them creatively in that setting. And he envisioned Disco as an anthology show that would focus on different eras each season, so the Burnham arc was one season, on to the next.

A lot clearly changed even before the show went into production, at which point he was out and Paramount probably reneged on doing new casting and design work for each new season. We’ll probably never know what could have been, and perhaps an anthology show would have the same dip in interest as it moved on.

For what it’s worth, the jump between seasons 2 and 3 did make that kind of radical change in setting that an anthology sets out to — but preserved the characters who had just fulfilled their mission to hide the sphere data, so there’s a contradiction in terms. And more to the point, the writers didn’t seem to know what to do with the characters once they made it to the future.

The evolution of Zora was an inspired idea (and literally cripped by Michael Chabon’s Calypso) but only became a detached plot strand, and Detmer’s PTSD was a gut punch only dealt with too superficially. So you’re right, despite some character highlights season three was meandering and listless. The crew had a whole future to explore, but no mission.

Rebuilding the Federation should have filled that hole with direction (or at least directives) but there wasn’t a lot of purpose to the space UN once it was restored. Maybe that dead water feels so frustrating because we’re seeing its literal mirror image in the deterioration of diplomacy and parliaments on the news every day. When Disco gets political it doesn’t mess around, but here it couldn’t deliver a show of common purpose because it was barely coherent itself. But I digress.