Great that it works. Auto-login is actually after the boot process, so I think this cannot be achieved easily. That it’s a few seconds black screen instead of seamless switch to and from splash screen isn’t perfect. But I don’t know much about it, so we’d need to read into it. E.g. not sure if one part is a pure kernel feature or everything manually loaded from initramfs/init, and if so what the initramfs is used for (probably allows optionally showing the splash screen earlier).
Does this change something?
update-initramfs -u