1.) Keep 3.20.6 and download the plugin from…
https://sourceforge.net/projects/hplip/files/hplip/
…this one worked for me, but…
https://developers.hp.com/hp-linux-imaging-and-printing/plugins
…Edit:…works now too.
Don’t forget to install manjaro-printer for a troublefree setup.
2.) Change to testing and be on 3.20.9 (like arch)
Ah forgot, welcome !