The most secure app I know is SimpleX. It is like Matrix but without an identifier, so you really interacting only with people you want. No one can reach you unless you gave him a link to connect to you.
Two different people cannot be ensured they are talking to the same person since there is no identifier they can compare to.
KDE with GNOME design or GNOME with KDE functionality.
Consistency between all elements, apps and other things.