So, out of curiosity, I came back to the legacy reader to have a quick comparison with the current one and see what's the pro and con we can learn from it to incorporate into the default one. Here're my thoughts:
1. Image loading: The legacy reader loads image piece-by-piece, whereas the default one waits to download the entire image and display it all at once. With slow connection, this is really irritating because the legacy reader allows you to slowly read the image as it loads (especially useful with 4-koma), whereas the default one forces you to wait till the image is fully loaded. If it fails to load, you have to tap on it to reload and waste more time waiting.
2. Page jumping: Some readers (myself included) may prefer the drop-down menu to select which page to go instead of entering it manually for several reasons. This is only possible on mobile and vertical screen, otherwise you have to tap between the pages and enter the page to go to into a prompt box.
3: Navigation bar position: While it's possible to show/hide many elements, the fact that it's located on the right side of the screen and squish the images when expanded is really inconvenience. Good to hear that it'll be relocated to the top of the screen on v5 (or we have the option to), but I'd like to point out that we should have the option to keep it always on, as this is an issue on mobile too (see below).
4: Mobile inconsistency: The navigation bar acts like it should on PC and has its own horizontal mode when used on mobile with the screen in vertical. The problem is that it hides lots of info, such as number of comments, and it isn't always-on so we're forced to jump back to the top to use it.
if anyone wants to add in anything else, lemme know so I can edit my post.