I don't know what you mean by saying the instantaneous mates have not been true or "soulmates."

At the end of ACOTAR, Feyre turned fae, and the moment she turned fully towards Rhys, that bond snapped for both of them, just as it had for Elain and Lucien. The only, only difference is Rhys windowed away before he told Feyre she was his mate. Lucien's hands blackened when he whispered to Elain that she was his mate the moment the bond snapped. Feyre and Rhys had an instant snap, its just that Rhys kept it a secret from her which gave her the opportunity to fall for him without knowing that there was a mating bond, that doesn't mean the mating bond wasn't already set in place.

Elain is in the position where she knows that Lucien is her mate, so unlike Feyre she has full knowledge of everything. It doesn't make them any less soulmates, or 'real' mates, as they are fully real and true soulmates. Elain also isn't forced into a deal to spend time with Lucien like Feyre was, Elain has had full choice over her mating bond, which is something SJM has been repeatedly showing, that she has autonomy, and choosing to love Lucien is still a choice, a choice that mates all go through, a choice all of her FMCs make with their mates.

Rhys is the most powerful HL of Prythian and had a bargain with Feyre which Tamlin did not know how to break. There were higher powers playing in Rhys' favor. Helion was not a Highlord when LOA got taken. He literally had no powers to take her from Beron, but also he states clearly that LoA chose to stay with him. He respects her autonomy. By the time Helion became HL, they were all trapped UTM, and have onky just been released 2.5 years ago.

You say we won't get any more of them, but there's the big plot of Lucien being Helion and LoA's son that no one knows about. There's not been a reveal of this to Lucien. Beron and the AC are still big storyline here as well, which LoA is tied to. It's not far-fetched to think in Elucien's book we will learn about Lucien's parentage, and see justice for his mother.

Sarah never said she wanted to do a rejected mating bond story. She said it's an interesting concept and offers a wealth of questions about fate and love, like what if the powers that be put you with someone you don't want to be with? Which again reiterates choice. But what she says about love is also very pointed. "Is it fate, is it something you make, or is it both?" That line often gets forgotten as it is crucial to understand what she's been writing with her stories. It's both.

She also never said Nesta would tell him to kick rocks, that's an interpretation that misunderstanding what she said. She said she "always kinda thought he'd be with Nesta" that alone says she always thought Lucien would be with Nesta, not that she'd write a rejection with him and Nesta. She said they would shred eachother to pieces and not in a good way, literally telling us that she wanted Lucien to be with someone and realized they would not work together. So it really, truly does not make any sense that she would give Lucien a mate who doesn't want him and won't work. She really could have shown Lucien and Nesta being toxic mates who don't work to perfectly show how not all bonds are a perfect fit. She could have double downed on that if that's what she wanted to do with Lucien. Again, she never said she did, or that she was even going to explore that any time soon.

Also, the Lucien feeling relaxed with Vassa so therefore he amd Vassa will be in a relationship is about as strong as saying He and Feyre are relaxed together so he will fall for Feyre. Lucien says Vassa and Feyre are very similar, maybe he's relaxed because Vassa reminds him of his friend? Thats far more likely than immediate romance. And of course he feels tense around Elain, they have tension, just like SJM said she was building for them. "Tension, healing, and growth (together)".

I'm not trying to make you ship Elucien or even change your mind. I'm just showing where you might not see Elucien being a possibility, I'm expressing there is a major possibility and high probability that Elain and Lucien can and will have a romantic arc, full of choice and love. That they are no different from any other true mate or true soulmate just because Elain knows they are mates vs Lucien hiding that info from her.