> i forgot that eBookJapan is barely working bc of outdated urls + CDJapan doesn't always show related volumes...
Well, that's easy to explain.
EBJ was sold out not so long ago, of course URLs changed. For some series old links properly redirect to new site. For some series it doesn't redirect. Feel free to report these or change it yourself.
CDJ was always weird in this regard. There's series pages for titles with 3-5 books. There's NO series pages for titles with dozens books :/
If there's no series page then you don't have other choice but link to single volume.