which is fucking useless for actual progress sync of books because it doesn't handle concurrency (multiple readers reading the same content, potentially offline), and more importantly, modern ebook formats have no concept of "page" in transit. Oh, you read page 10? Awesome! Now do tell, is it page 10 on a 5" 800x480 eink display with 48px font size and giant margins/lineheights/word paddings, or is it page 10 on a 13" display of 2480x1860 resolution with 11px font size and barely any margins? Since you'll get wildly different results in both cases, and OPDS doesn't really allow for adapting this simple integer to a precise position.

No, for that you require a proper locator scheme, something OPDS doesn't provide and cannot enforce.

Page based progress is fine for fixed format publications - comics, PDF/DOCX files, etc., but that approach breaks irreparably the moment you switch to dynamically formatted content. In case of EPUV/MOBI/the various Kindle formats, you want to determine the reader's position based on the first and last paragraph/sentence visible on the reader and correlate that to a position within the actual files of the book, which is actually dynamic, as it can be resolved regardless if it's XML formatted EPUB or if you dumbed the book down to a simple TXT file.

So no, OPDS's PSE is at best a stopgap solution for syncing progress.