Hi @jdloft! Welcome to the community! Please have a look at the introductory posts in the #start-here category if you haven’t had a chance to do so.
Have you rebuilt your initrd image after changing plymouth theme? As far as I understand, you need to in order for change to take effect.
$ sudo plymouth-set-default-theme tribar -R
Note that the -R flag will rebuild your initrd, and the next time you reboot your system, you will see the new theme in action.
Here’s more details about changing plymouth themes in a Fedora magazine article.
As far as I understand, the answer is “maybe”. A pair of quotes from Fedora FlickerFreeBoot wiki page:
Please keep in mind this is still a work in progress.
Take a machine with i915 graphics (amd/nvidia graphics will still see the monitor turn off briefly for now)
Also I can say that on my notebook with older i915 graphics and encrypted root, /home and also temp partition, the boot process is quite close to flicker-free when I reboot my machine (and I see a graphical LUKS passphrase request on after reboot), but on an initial powerup LUKS passphrase request is in text mode. I think that’s because I have my temp partition encrypted in addition to root and /home as that’s not the default. But I haven’t tried to troubleshot it.