Ds9 was the high water mark of Trek as far as I'm concerned.
I halfway agree. DS9 was great because it was
different than the rest of the show, and it's darker tone let it explore subjects in ways the others could not. TNG's "I, Borg" and "In the Pale Moonlight " explore very similar concepts but in totally different ways, because Picard had the luxury of looking at the issue in a pure abstraction with any potential consequences merely that, potential issues for another day, while Sisko knew that every day that went by without the Romulans involved carried a cost in federation lives and dwindling odds of victory.
But the key point is that it's only great
in contrast to the rest of trek. Take away that context and it's generic gritty space drama #30, that will be vaguely notable but not really memorable or consequential (when was the last time you heard B5 or Andromeda being discussed?). On it's own merits I don't think it's nearly as good as it as as part of the franchise.
After DS9, they couldn't get people to care about the characters anymore.
While that's true, I think you're attributing a bit blame to DS9 there that it didn't deserve. VOY and ENT just had bad writing that undermined many of thier characters, there was nothing about being post-DS9 that did that.
And frankly, DS9 was not so much great with characters as it was just OK with many of them. O'Brien was every crusty old "don't call me sir, I work for a living" old soldier NCO type from every other show rolled together, Jadiza was so overwhelmingly mediocre that when she was replaced in the very last season her replacement, who was only there for one season, was so great there's still debate over which on was better despite the very short amount of time we had with Ezri, Garak was so vague and mysterious that people in universe called him out for intentionally deciding to be vague and mysterious rather than develop his actual backstory, and Odo spent
five years wrapped up in a "will they or won't they" plot with Kira.