You know why I can buy Carl and Bryce both being into Aniya and Trinity fairly easy but struggle to really accept that Zach is super into Kayda? Zach doesn't like to embarrass himself for love, and you can tell.
With the islanders consistently telling us Zach is crazy about Kayda but us seeing how Zach moves, I think he hates the idea of being embarrassed too much to fully commit to being a good partner.
He hates the idea of being rejected by Kayda, but he clearly did like her pretty soon into the show. So when she agrees to chat with Caleb because he isn't willing to put his foot down and say he wants to close things off (arguably because he just also wanted the full love island experience with casa amor exploration) it means that when she similarly explores because apparently they're still open, suddenly he gets moody and territorial. Rather than being emotionally present and talking it out properly like Bryce does with Trinity, Zach makes it her problem and punishes her when she moves the same way we've seen him.
Like he's had chats with other girls, notably I remember him talking to Jen and even slightly flirting with her after he got upset at Kayda for talking to Caleb.
Despite connecting with and having fun with Alannah, it really doesn't seem like he would've brought anyone back even if she hadn't been kicked. Because the second he realized he could lose Kayda to someone else he got really huffy and upset about it, and I can't even say that's because he's hating the idea of any girl he's with getting taken away. Because with all the Casa Amor girls if he really wanted to connect with one of them and bring her back he had all the room to do so.
Zach doesn't like owning up to the bad things he does, he doesn't like taking the knee and making himself a little smaller for the comfort of his partner even momentarily. He like's Kayda but he dances around being actually vulnerable with her. Meanwhile Kayda is always open to him. She's always ready for the next step.
I do think Zach wanted to win, badly, but he wasn't willing to take the steps to really endear the public to him in a way that would've cinched it. Fans have forgiven messy Casa amor experiences before, but Zach wasn't willing to do the work required to properly come back from it.
I believe the islanders when they say Zach is obsessed with Kayda, because any time a man made a move on her he got really upset and ran around causing problems over it. Remember Bryce pulling Kayda, we can't pretend him getting vicious and telling lies about Bryce to KC wasn't in some way motivated by him being mad Bryce was trying to yank her. And I only bring this up because once Bryce actually locked in on trinity, Zach was one of the guys affirming him at Casa.
Zach needed to be honest and bare himself, and he wasn't willing to do that. If he continues to be unwilling I don't see him and Kayda lasting. If he's not up for being cringey and accepting when he's wrong with grace I can't see them staying.