I've been wanting something like that: a manga equivalent site to Webtoons/Tapas. As somebody who working on a manga and converting it to a webtoon format, I feel like the storytelling methods are completely between the two. With manga and traditional comics, there is a lot of attention to the pagination. Pagination is the placement of pages to allow for flow and story telling. You would have to consider the last panel of the spread and think about how to a) entice readers to turn the page and b) how does turning the page impact the feeling of the next panel. It creates for some dynamic moments, comedic moments, or suspenseful moments.
That said, webtoons/tapas format benefits from it's own method of story telling. Since the comic is in a scrolling format, it's more suited for scene by scene storytelling similar to that of a storyboard. It allows for movement in the scene and works for formats that involves a lot of movement.
While I wish there was a site for manga in the same vein as webcomics, I feel like it may be a difficult endeavor. For one thing, manga format stories are a bit of a niche. Webtoons is a popular format due to it's accessability; you can read it from your desktop or your phone. Manga format does not translate well to a smaller screen so it will be difficult to capture as large of an audience.