EDIT: I found my favorite PS1 animation helping out below! HERE it is!
I have all of the retro consoles boot animations that people were cool enough to switch up into a Steam Deck boot animations I also have the plugin for deck tools that allows you to get a random one after each reboot. Needless to say, I never get tired of hearing the old OG Gameboy, PS1/2, GameCube, etc. boot animations and sounds. Core memories indeed!
It never gets old! I have to boot into Windows partitions for some few things, so coming back to these retro boot animations reminds me that SteamOS is home. :P
Animation Changer (I recommend just following the install guide for Decky Loader, opening the plugins shop from the side menu, and then finding the Animation Changer plugin. I just included this link so you would know what to look for, and to see some of the dope ass animations people have been kind enough to share.)
Happy hunting! If you don't know, you can also install CSS themes, change the background noise and UI audio elements too! I've got mine with a Fallout (Blue) theme, sounds from the Pipboy for navigating the menus, and most of the retro console boot animations. :)