On the subject of Kairi I do think KH3 improved her treatment a bit even if she still can't pass the bechdel test and was still somewhat shafted. We got to see Kairi have a friendship outside Sora and Riku. We got to see Sora and Kairi's relationship advance including them finally having their long awaited world tour together. We got to hear Kairi's actual thoughts and feelings for one on people and not just Sora (namely Namine). Like it was a particularly nice piece of characterization hearing Kairi write a letter to Sora but then explain that letter isn't actually for Sora, it's for her a, a coping mechanism she uses to process her thoughts because Sora is her happy place where she feels most comfortable. We got to play as Kairi for once and fight and defeat MX, the Xehanort saga got to end with Kairi beating Xehanort then joining the group as they all did the final blow together.
She got have like flaws the start of an actual arc. Promising to protect Sora and holding her own against the generic mooks, but freezing up in her first Xehanort fight which is understandable. Easy to forget when Sora first faced a terrifyingly seemingly powerful foe he turned around and tried to run away until he realized there was nowhere to run to, Kairi's reaction was pretty fittingly in the middle of Sora and Riku who were flight vs fight. She still managed to make a come back and one-up her only meaningful contribution, the saving of Sora in KH1, to now have saved Sora and by extension all the guardians as she is the only thing that kept him tethered enabling him to save the others.
She reaffirms that she will keep Sora safe and from that point on she held her own even against Xemnas until she got caught and fridged. However the fridging wasn't just a way to cause Sora pain it set up the entirety of Re:Mind, this game, and basically is the catalyst to the core plot going on. It was a mistake, an actual failure, with serious long term consequences. I wouldn't have done it this way myself but you know this is something everyone but Kairi has had. A tragedy, a mistake, a personal failing that affected everyone including the people closest to them in a negative. Kairi promised she protect Sora and instead Sora had to save her and was erased as a consequence. That had to hurt, to give her such a complex knowing that Sora is gone and no one's life has been able to return to normal and move forward because they all have spent the last year trying to save Sora. Kairi finally gets to join Sora, Riku, Terra, Aqua, Ventus, Roxas, Axel, Xion, and Namine in being a character with flaws that lead to failure and long lasting problems. Instead of the perfect do nothing pure maiden whose effect on plot and on people has been minimal to non-existent.
Finally as pointed out Kairi didn't just return to the island and wait. She went to RG to try and use herself in their research to find Sora doing all this melody of memory dive stuff, and who knows what else since we know at some point she is going to end up in TFW staring down a cloaked figure. Point is that she IS doing things and not just waiting...even if her and Riku's doing things to find Sora amount to playing video games where they play as Sora.
Can and does Kairi deserve a hell of a lot more than this? 100% without any single question yes, but I think it's often downplayed how much better KH3 and onward has been treating her. It's still not anywhere close to good enough but compared to where she was previously? The distance between her treatment in KH1/KH2 vs KH3 is like the distances from north pole to the south pole.