My experience with the linux boot has never been flicker-free. It’s bugged me for years, but I don’t have the technical knowledge to fix it. There’s a black screen between BIOS and plymouth, then a black screen between plymouth and the login screen, then another black screen between the login screen and the splash screen, and finally a black screen between the splash screen and when the desktop shows up.
Mac and windows do a much better job at having a seamless experience from boot to desktop.