There’s another manga by the same author about an antique shop that only appears during ...

I think it's already uploaded here. It's not the same author though.

It's this one: https://www.mangago.zone/read-manga/ugetsudou_antique/

I don't know why mangago tagged the manga about the antique (Ugetsudou Antique) on a different author but on other reading platforms, the author of this story and the story Ugetsudou Antique is the same. I also did a quick research and it says that Fumi-sensei is indeed the author of Ugetsudou Antique.