Zenphone 2 https://forum.xda-developers.com/zenfone2/general/guide-brick-soft-hard-bricked-zenfone-2-t3284256

Magic Device Tool https://github.com/MariusQuabeck/magic-device-tool

F-Droid https://f-droid.org/en/

K-9 Mail https://f-droid.org/packages/com.fsck.k9/

Protonmail https://protonmail.com/

Signal https://signal.org/

Syncthing https://syncthing.net/