I've never liked the idea of the Rygart x Sigyn ship and I never warmed up to it either. Personally, while I do acknowledge that the feelings they hold for each other are genuine, I think it's far too late for them to pursue any type of romantic relationship now. In the years since he dropped out of school, while their overall feelings towards each other may have changed, they've found themselves in various circumstances where pursuing a romantic relationship is no longer appropriate and can even be seen as unhealthy depending on who you ask.
Despite the fact that it was done for political purposes, Sigyn still married Hodr and even tho this was resolved with them getting a divorce, I still think Rygart shouldn't do Hodr dirty like that. Idk man, even if they weren't a legitimate couple and were only married for the sake of keeping appearances, something about it just doesn't feel right.
Of all the women Rygart currently has any chances of pursuing a relationship with, I think the one who'll be most helpful in keeping him sane and helping him deal with his PTSD is probably Narvi. Being a fellow soldier, at the very least, she should know better than Sigyn what he's feeling/thinking right now. We've got glimpses of it throughout the manga as well where she's usually the one to notice the subtle changes in his mental state. However, the issue with Narvi is that she doesn't get much screen time in the first place and her shared panels with Rygart are even scarcer. Regardless, thus far I still view her shared moments with Rygart as more important than the majority of his shared moments with Sigyn.
I don't deny that Sigyn also understands a part of him that no one else can, but she can't really help him when it comes to dealing with these particular problems and it shows throughout the manga. All she can really do when he hits rock bottom is apply a "bandaid" on the issue to slightly relieve some of his suffering.
Tl;Dr: Imho, the healthiest potential relationship I've come across in this manga is Rygart x Narvi, but the author seems dead set on going with Sigyn. It is what it is, I won't hold any grudges if she wins.
I just felt like sharing my thoughts/opinions on the shipping in this manga.