Honestly, if the manga ends any time soon any relationship Asumi ends up in will be disappointing.
As it stands, while they at least have a relationship, Nanao and Asumi's dynamic is... not particularly healthy. Nanao is clearly suppressing her jealousy, and the relationship as far as we know is one-sided, with Nanao deeply in love with Asumi and Asumi essentially taking advantage of her feelings for sex. There are hints that Asumi may return her feelings, but there's just as many hints that she doesn't, and at the very least is currently unsure.
As for Mai, I mean, her introduction involved her fantasizing about child-Asumi. What more really needs to be said about her? The fact she hasn't shown up for 90% of the manga doesn't help her case, because she's not had a chance to develop out of that rather unpleasant first impression.
That all said, I think if you've read up to this point expecting Asumi/Nanao to be endgame, you're probably gonna be disappointed. The premise of this has always been 'protagonist searches for her long lost first love and they miraculously find eachother like Destined Lovers.' I expect we'll get a volume of Asumi/Mai like we got of Asumi/Nanao, maybe a volume of wishy-washy "I don't know who I truly love" plot, and then arbitrarily choosing Mai at the very end.
...That became a bit more of an essay than I meant it to be.