I wrote a little tool for creating splash screens: https://github.com/basxto/fp3_splash
The first image was taken before I fixed a bug that cut off a part of the image.
Just flash them like
fastboot flash splash ./splash
Here a few splash screens for testing:
https://basxto.de/fp3/splash/splash.hoe.img (FAIR HOE)
https://basxto.de/fp3/splash/splash.unbricked.img (UNBRICKED EDITION)