I think sadly the reason why all TNBC sequels never work out is because they always choose one character to focus on. Like they pick Jack or Sally to focus on and the other is just there as a side character the main character a has to save. In the movie it is both equally there story Jack just gets more attention then Sallys dose sadly. One cannot survive without the other and that’s why every sequel is inadequate because it just chooses one of them to tell the story tell. They make a story where both Jack and Sally can be together in an equally story we won’t have a decent sequel.
I share your sentiment; the reason Zero's Journey works as one of my favorite TNBC sequels is because, while it's a tale about *Zero*, we get to see how it affects the other characters and watch as they step into the story for themselves. Lock, Shock, and Barrel have some great time in Christmas Town with Santa Claus, we get to see how Zero's absence is affecting Jack and Sally comforting him in this time, and even a spotlight on the Halloween Town Residents as they join in the search for him. I can't rave enough about ZJ in a time when LLTPQ got two unnecessary sequel books.
Speaking of LLTPQ -- I've had a lot of discussion lately about the series and where it stands now. My feelings are complex, but ultimately, I think Disney is losing its footing with Sally-centered sequels because they're meddling *too* much in what they want and don't want in these stories. I think they were more lenient with the initial Long Live the Pumpkin Queen because it was among the first of Sally sequels and experimented with quite a few things. We have Jack and Sally getting married and going on their honeymoon, Queen Elizabeth II, and other things that challenge what we've gotten in other TNBC adaptations.
Now that Disney knows Sally sells *really* well, and LLTPQ is arguably the best performing TNBC book, they're more comfortable with telling the author(s) that she CAN'T have children with Jack, they're more comfortable with them making up random towns for Sally to visit and new, nonsensical villains instead. Even when the author's vision was to have the element of time also consist of Sally's marriage with Jack and having children with him + his desire for such a thing as well.
They want to play it safe with Sally. They want her to be like all the other modern Disney Princesses and be 'badass', inspiring, always the hero and saving her love interest, because they abandoned the alternative awhile ago. And it's so funny to see the author(s) keep writing Sally expressing that she clearly doesn't want this, she just wants to be with Jack more than anything else, even imagining having children with him. Sally is strong and inspiring in her own right, but she's also a lover and a dreamer, and I highly doubt she's the right fit of a character for a three-book series on saving Halloween Town again and again.
As you said, there's also been a struggle to have Jack and Sally serve as equals to each other. And having Jack being the damsel in distress/in peril instead of Sally doesn't do a lot for equality, despite how much Disney thinks it does. I'd like to see them progress in their marriage, as Pumpkin King & Queen, adventuring together like was promised in the original LLTPQ, and talking about how they might move forward in their relationship (i.e. children). But for some reason...we just can't get any of that??? Only ONE book allowed Jack and Sally to go on a journey together, and since they weren't married, it was full of crushing and mutual pining. I'd like to see the husband and wife *DO* things together!