For uploaded books I would find it very useful if there was an option to display page numbers on the bottom of the page instead of a percentage. That way I can count pages more easily, and see how my reading speed is progressing.
Also it would allow me to flip through a book and get back to the page I was before (this is probably easier for you to develop/implement than a bookmark function).
The challenge with page numbers for uploaded texts is that Readlang repaginates content based on your screen size and font settings, so the page numbers wouldn’t match the original book. The percentage gives a consistent measure regardless of display settings.
For navigation, you can click on the progress percentage at the bottom of the reader to access “Go to start”, “Go to furthest read location”, and “Go to specific location…” options - this feature is a bit hidden but might help with getting back to where you were.
Also, your current reading location is stored in the URL in your browser’s address bar, so you can use your browser’s bookmark feature to save specific locations in a text.