Well it is 🤷.
If he goes on a family day out, there'd be his new partner (when he gets one) and any DC his new partner has, your shared youngest DD, his eldest DD - and if he includes your eldest DD too, that's a child belonging to his XW who his eldest DD doesn't get along with and who his new partner and her DC have no interest in or attachment to either. It doesn't work. Your eldest DD isn't his DD, so that's where the dividing line goes, she's on one side of that line with you and he's on the other side of that line with his DC (when his youngest isn't with you).
YABU about DSD. She's never lived with you so she's had no need to form relationships with your or your DC. She's attached to your younger DD because she's her sister. It's pretty simple from her perspective.
My sibling has a stepchild. A spoiled brat enabled by their parent, who doesn't treat my sibling with basic respect. I DGAF about this child, they're not my family. My siblings DC from this marriage and the previous relationship are my family, we share DNA, I've been part of their life from the beginning. When I ask my sibling about the DC, it's those two I'm asking about. I'd feel warmer towards the stepchild if they were a nicer person and if they were willing to engage with wider family get-togetgers, but my sibling's partner and stepchild have always chosen to absent themselves. They've been married for years. I've met the partner and stepchild only three times and one of those was the wedding. Why would I be inviting this stepchild, who is nothing to do with me, on a day out? Why would I be gifting them anything? It would be awkward and feel complicated.
YABU. He's not going to be bringing her sweets and magazines etc because he's made it clear to you that he's not interested in maintaining a relationship with your eldest DD. He's not coming in because it's clear to him that you haven't explained to your eldest what splitting up actually means for the family dynamics and her place in things. Instead of being a parent and helping her navigate the situation which you brought about, you're letting her get hurt by trying to continue a relationship with him and being rejected and setting up a situation where you're blaming him for that and probably hoping that she does the same.
He's doing what people who are single do, putting his DC and himself first. Which is natural and normal.
You broke up your marriage because you didn't like the inequality between your DDs. Fine. But you surely can't have been mad enough to think a split would improve the inequality?! It was only ever going to make the division worse and cement the fact your eldest DD isn't part of your XH's family.
What did you actually expect from divorce? Your ex already has two DC from failed relationships to financially support, I can totally understand he doesn't want to voluntarily take on a third DC he has no legal responsibility for. I'm sure he wants some of his earnings to go towards his new relationship when he gets one and any subsequent DC he has. Your DD is your legal responsibility. He's right, it's far to complicated trying to maintain a relationship with your DD whilst ensuring there's no legal responsibility placed upon him to financially support her.