If you know of any FOSS, offline editors for Android which can do what silverbullet can, drop a link.

I use it and love it. Having the metadata (tags, dates, …) of your pages available to query and organize is awesome. I also love the tagged tasks feature.

Not that I know of. I think this needs to be implemented in a system framework.

Basically, yes. IIRC the FIDO/U2F handling is implemented in google libs.

They are however re-implemented in microG: https://github.com/microg/GmsCore/wiki/Implementation-Status, so if you are open to using that it should work with apps using the corresponding google client libraries.

I have used Firefox on Lineage/microG and tested FIDO2 there, it does NOT work with Fennec as that does not include the required client side libraries.

I use nextcloud.export which comes with the snap (https://github.com/nextcloud-snap/nextcloud-snap/wiki/How-to-backup-your-instance), but the backup app looks nicer.