i love your explanation, it's what i also believe in!! i wasn't expecting yuuya to have had a past girlfriend until inori was introduced, but i'm actually always really happy when she appears (more so during when they have past recollections, those are some of the few moments you can really notice yuuya looking more vibrant--besides the nukos, inori's the one who makes him blush and smile so naturally)
they're both waiting for the day that they meet again, and as you said, it's not a toxic way of thinking at all, because inori wants yuuya to cherish his life the same way that yuuya wants to, until such time that he passes on (NATURALLY) and they're finally able to be with each other once more
Also this is in response to a comment 1 year ago, but I don't think Yuya is stuck in the past or needs to move on from Inori. It's fine that he wants to keep her as his only girlfriend as long as he continues moving forward w the rest of his life, which he IS doing. He explicitely mentioned at one point that he's so happy ahout having his new family that cares about him. It seems more like he wants to live life out how he wants and when he reaches the end naturally, there'll be the love of his life greeting him. It's NOT like he wants to die as soon as possible to meet Inori again. That's clearly super toxic. But it's not the case for Yuya and this story.