Dune was my favorite series until TBP. Dune degrades quickly after the first book while I thought each TBP was better than the last.

Dune holds up way better if you pretend it's just one book (the first one). It stands alone very well and IMO is completely worthy of its status in the classic sci-fi pantheon.

On the other hand, I finished TBP, but I disliked it enough that I didn't go back to the other two volumes. What makes them better than the first, in your opinion? (purely for my own curiosity re: whether I should go back and read them)

The first book is more character driven and the plot moves slowly, glacially compared to the next two. I don’t want to ruin the plot, but I‘d say they exceed Dune’s vastness without the long grinds.

The author described how he approached the series as wanting to write about the “the worst possible universe”.

I recommend you read books 2 and 3 in the REP series. TBP is comparatively really slow (IMO necessarily), but the series really picks up in 2 and 3 in a way that makes you appreciate the foundation that book 1 laid.

After reading all three, I often think back on book-2, sometimes on book-3, never on book-1.

The first book’s the best one but there’s an arc that carries through to its conclusion in God Emperor that kinda completes the whole thing.

I did read 5 and got about a third of the way through 6 before stopping and I still don’t know why either book exists. The previous story was over, and the new stuff’s muddled and not compelling.

5 and 6 are bizarre; not only do they read like action thrillers, they almost seem to contradict the whole message behind the Golden Path. The series should absolutely have ended with God Emperor (my personal favorite, followed by the original)

I'm amazed these are being spoken of in the same breath. IMO Dune is vastly overrated

Ok, but I'm also amazed, because I found TBP to be vastly overrated...

To each his/her/their own.

And I love both of them.