Video DownloadHelper:
https://v10.downloadhelper.net/
The external program this browser extension relies on also supports Linux :)
cat-catch:
https://github.com/xifangczy/cat-catch
That'll cover almost all direct from the website rips.
Cat-catch is for websites that make it hard to capture a video stream. Especially premium/paid sites.
e-hentai.org is not just japanese but also western adult comics. They have direct download and torrents.