It’s never too late. “Back then”, when I started using Signal (called TextSecure), only one other single friend used it. Nowadays, almost all my personal contacts use it. Every additional Signal user adds a contact in someone other’s address book as a potential Signal contact. It just takes time. Good luck!
Aim for a more open solution, like a Tolino or Kobo.
Pocketbook runs Linux, IIRC.
As far as my experience goes, all you can do with Gnome Software is to install and uninstall software from various sources which need to be added separately. There’s no option to manage any remotes/sources even with flatpak support installed. I hoped to be proven wrong.
How do you manage flatpak remotes with Gnome Software?
Related app, but which can also be used without Trakt: Series Guide.