We use the Web Annotation Protocol to sync bookmarks and last reading position across devices. At a glance it covers all the use cases here, and it's a well-defined protocol with multiple independent implementations. In particular, WAP defines a relation for discovery. Here's how we link to the annotation endpoint for a specific book in an OPDS 1.2 feed. Note the distinctive link relation and media type:
xml
<link href="https://circulation.librarysimplified.org/NYNYPL/annotations/Gutenberg%20ID/40278/"
type="application/ld+json;
profile="http://www.w3.org/ns/anno.jsonld""
rel="http://www.w3.org/ns/oa#annotationService"/>