I just thought Season 3 was one of the most darkest seasons in all of Star Trek and that includes DS9. It just didn't feel like Star Trek to me, but upon rewatch, I think I left my expectations at the door and it did get better.
I liked the grittiness of it... but I'm also glad that it lightened back up again. It felt right during that arc that nothing was rainbows and lollipops but it was also good that it had an end and chasing the Xindi didn't become the end-all-be-all of the series.
Welcome and enjoy.