Just keep in mind YUKI is no longer actively developed, and their git has been archived since last August. Should continue to work for a while. On not being in the Flatpak search, it is a Flatpak that is a user install, it looks like. Solus is setup with system integration. You can use both, but just why it does not show up in search as some "user" install flatpaks limit appstream data depending on the dev/packager.
Hypnotix does seem to be actively developed.