I think it's so easy for everyone to just think, "Bring her with him," without thinking of the multiple nuances within the story. Your last statement really nails it that no one who hasn't read farther can't see the big picture. I'll try my best to explain my thoughts without spoilers.
First off, the timing is terrible. Their mug cups are about to go into production and Tatsuki's job is basically finished. He created the shape of which the molds are modeled after. After that, the casting is done by another place entirely and all he'd really have to do if he wanted is inspect the vessels to ensure they match his model. But Aoko on the other hand is the painting team lead and is the primary person to do the slip trailing and training the others to do it. Her hand will somewhat be on every vessel, so her job is very important.
Secondly, she wasn't invited to go. In most instances if an artist is invited to some type of event, their travel and accommodations are sometimes covered by the invitee. It would make sense to me that since there are potters in Finland who wanna see his work so badly to invite him to exhibit it in a gallery, then I think his travel was somewhat comped. Even if not, Aoko's not the type of person to offer herself on a trip if she's not invited. And Tatsuki probably see's no point in inviting her because a month to him (or anyone) isn't that long.
Third, their relationship is still fairly new. I cannot speak for everyone, but if my partner had to go on a business trip only after a few months of dating, I will not offer myself to go nor would I expect them to offer. Now, if we had been dating for some time then obviously that could change.
Fourth, I don't think people realize how truly traumatizing it is for someone you love and care about a lot to literally up and disappear for reasons you cannot understand. You are left with the promise that they will return and they love you and will do anything to come back, and they just don't. I'm sure that sadly many people can relate when parental figures do exactly this. It impacts you life much more and for much longer than you'd think.
Lastly, and the biggest one to me that everyone seems to forget, Aoko has NEVER BEEN OUTSIDE OF HASAMI! This is pivotal to her character! The farthest she has gone was to the Kyushu museum but that is it. for 32 years she has stayed in the Nakaoyama region and never left. Now you expect her to just up and leave? Besides even that, don't you think there'd be some important missing things that someone who's never left the country would need? 👀
Sorry for the BIG YAP but ya'll gotta think BIG BRAIN HERE!
I give thee my southern card
