How is it possible, that Signal still only provides a .deb package and no .rpm, or even better AppImage or Flatpak? There is an unofficial Flatpak but is it secure?
OP, what distro are you running? You mention a whole bunch of package formats they don't provide, but never mention what format you require. Depending on the distro, making a build script (or converting the .deb) really isn't Rocket Surgery ™.
What percentage of Signal users is "grandma" that uses Linux and would be messaging from her PC? I would have to imagine the overwhelming vast majority of Signal users are on mobile only, so packaging for specific distros is probably far down the priority list.