Turns out the WatermarkPath option I was using is a downstream patch that isn't usable for anyone else 🙈
$ git clone https://src.opensuse.org/fl4nn/plymouth-theme-xenia.git $ sudo cp -r plymouth-theme-xenia/xenia /usr/share/plymouth/themes/xenia
$ sudo plymouth-set-default-theme -R xenia
Xenia image by @Neotheta@finfur.net, downloaded from Fur Affinity.