[pazos]

I downloaded it from the Boox app store

That's a rip-of, old version, distributed by a known piracy advocate company. hey do not comply with the AGPLv3 license this program has.

You can download the proper version from F-Droid or from Github.

Also see install instructions for android

However, I've not been able to set it as the default reader app. I've done the long-press on a book title, selected 'open with' and the only option is the default Neoreader.

Onyx launch activity might be non-standard. KOReader should be shown in the app picker for the following formats: https://github.com/koreader/android-...st.xml#L49-L90

Try with a non borked app copy. If that still doesn't work you might try to get luck in Onyx Boox support forums