This app was mentioned in 4 comments, with an average of 1.50 upvotes
Kontalk has a weird name and the app doesn't look good. They need to work on improving the UI and maybe changing the name to something that makes sense. Aside from that, it seems promising.
Not sure if this has all the features you want (search in its settings to see if you can disable online/last seen and other features). It's Open source, uses end-to-end encryption, looks simple and good.
F-Droid link: https://f-droid.org/repository/browse/?fdfilter=Messenger&fdid=org.kontalk
Play Store Link: https://play.google.com/store/apps/details?id=org.kontalk
Have you tried Kontalk?