Just finished the Jadzia show, and I think you're right that the character is unfairly maligned. I have often been one of those who considered her one of the least developed characters on the show, but you did help me to see how there's more to her than people realise. It's just that her character development is not as obvious
as people like Odo or Kira.
A couple of extra angles to discuss that you didn't have chance to get to. 1: her relationship with Quark. She's one of the few people on the station to accept him just as he is without trying to insult him or Ferengi culture in general. It's interesting that she has such affinity for both Klingons and Ferengi, even though those two cultures have generally been portrayed as diametric opposites. While Kira and Sisko and Odo and everyone else always rag on him and complain and never take him seriously, Jadzia just gets along with him. I think that speaks to her experience with all kinds of people over 300 years, and finding something to love in all of it. You're also right that she has a positive effect on him in ways he probably doesn't even realize - the thing that makes him most understand that he's crossed the line in "Business as Usual" is when even Jadzia is annoyed with him, when she's accepted everything else he's ever done.
And 2: I think Jadzia - or possibly just the concept of the Trill in general - allowed Star Trek to make its most varied exploration of sexuality. Not just in terms of the Lenara relationship, but also when you look at who is the "space stud" of the cast. TOS has ladies' man Kirk. TNG had ladies' man Riker. But DS9's most sexually active character is Dax - at least until she settles down with Worf. And the fact that she's a woman in that role is more quietly revolutionary than I think she gets credit for. I don't think anyone doubts that for the first four years or so she's a very sexually active woman - Captain Boday, the wrestler guy, hell she even asked Morn out - but never once is she "slut-shamed" for that. She's just the "space stud" of the show, same as Kirk or Riker... and she's a woman.
And I think that's not necessarily restricted to men. The fact that she was willing to build a life with Lenara means that obviously she would have been open to having sex with her at some point. Ezri may have been joking when she said "I don't think there's anyone on DS9 who wasn't her lover," but there's a kernel of truth in that - she didn't discriminate. The host body may be hard-wired with a certain sexual orientation, but since the symbiont provides access to memories of sexual encounters regardless of gender, it would inevitably lead to a certain fluidity of sexuality. And Jadzia as a character provides an opportunity to look at that in a way no other Star Trek character does.