Gah, so many feelings. OK. Personally, I'm a huge fan of completing the triangle as a romantic resolution, and more poly is always great to see, especially in fiction I actually enjoy reading. Plus, the image of Ling Qi and Sixiang engaged in a relationship that's queer on basically every level as Sixiang supports Ling Qi in the Husbando Wars, their relationship shifting between more platonic and more romantic as Sixiang shifts gender is just squee-worthy. Also, the prospect of future chapters where Ling Qi, Sixiang, and Ling Qi's future husband navigate their relationship makes me practically salivate in cozy anticipation.
That being said, I have reservations, and they are not small. To start with:
"But… I do think there are things I might like to stick around for… I just don't know if they'd be enough without you as my anchor," Sixiang said. "I was kinda… made for you, even if I've grown past that a little."
And there is was, a thing that caused her some discomfort. She knew Sixiang could see it on her face, too, the way his expression dropped.
Sixiang was a dream that came into existence based on what Ling Qi needed (or at least what the Dreaming Moon thought she needed). They've already grown tremendously beyond that, but it's a much better foundation for a confidant than a lover. If we put aside any ethical questions on the issue, that fact of Sixiang's existence also makes Ling Qi uncomfortable, but is fine with Sixiang, which would be a stressor on their already complicated relationship, as they struggle to see things from each other's point of view.
"One way or the other, we need to decide what we want to be," Sixiang said, shaking his head. "I'm not gonna compete with those guys, you know? That feels weird to me… nightmare-y. I know humans might think it strange, but I think you're cute with them. I don't know how any one of 'em would react to a plus one, though."
Even with how far she had come, Ling Qi felt very uncertain around the idea herself. She certainly couldn't even imagine entertaining the idea with anyone but Sixiang. But, all the same…
Sixiang brings up a good point here, we don't actually know how any of our suitors would feel about this situation. Ling Qi is being pressured to get married given her status as the heir's right hand, and she's considering it despite her issues with intimacy because it would be a major boost for her goals. Whoever she marries has to be an advantageous match, and has to support Ling Qi when Cai Renxiang rises against her mother. In addition, Ling Qi wants a close family, which means she wants a love match. She has a lot of criteria and something of a time crunch, in terms of bringing someone in early who can then start laying the groundwork for the revolution. Adding more criteria is not ideal. Also, on a personal level, Ling Qi is a bit of a stick in the mud when it comes to relationships. She's very accepting of others, especially people with different lifestyles, but she's shown to be very conservative in her own love life, even beyond her issues with intimacy. Her being uncomfortable with a poly relationship is, again, a stressor on her nascent romance with Sixiang.
"But… I'm not going anywhere either way. If you just want to be friends… be family, that's fine. I do… I got other things," Sixiang said. "So. What do we want to do?"
Sixiang finds the idea of not being in a romantic relationship with Ling Qi painful, or sad, or regretful (honestly, not enough to go on here to make a definitive statement), but they also acknowledge that they would stay either way and that they could live with it. I think they've learned their lesson when it comes to keeping their feelings bottled up after the tribulation with the Emerald Mourner, but I can still see easily see a future where Ling Qi and Sixiang pursue a romantic relationship, and Sixiang reflexively hides any negative emotions out of fear of losing this new thing with Ling Qi that Ling Qi isn't totally comfortable with.
Finally, on a personal note, I'm not sure a romantic relationship with Ling Qi is good for Sixiang. We don't actually know what's going on in Sixiang's head, but to a certain extent, it seems like Ling Qi is Sixiang's whole world. They have good relationships with Hanyi and Zhengui, and they enjoyed their time at the sect with Li Suyin and Xin, but Ling Qi is their deepest connection, and for most of their current existence, she has been "the only real thing that mattered". If Ling Qi and Sixiang get together, I can also see a future where Ling Qi is their everything, both their lover and their only true friend, Bloodfall toxic yuri style. Making a clean break here on the other hand, pushes Sixiang out to find romance elsewhere. Given that they decided not to return to the Dream, and have only continued to reinforce that choice every time it is presented, I can't imagine they wouldn't choose to do so in time.
I'm not sure if I got all my thoughts out, but it's as good as it's gonna get.
(Which incidentally is very interesting dynamic in that it sorta makes sixiang... temporarily very traditional in gender norms? "girl mode because ling qi was leading in the dance" and "boy mode because i'm being assertive to ling qi" being the two initial examples iirc). 
