In /boot/config.txt, add:
disable_spash=1
optionally:
disable_overscan=1
In /boot/cmdline.txt, add:
logo.nologo consoleblank=0 loglevel=1 quiet
By hand run
systemctl disable getty@tty1
apt install fbi
Create /etc/systemd/system/splashscreen.service:
[Unit]
Description=Splash screen
DefaultDependencies=no
After=local-fs.target
[Service]
ExecStart=/usr/bin/fbi -d /dev/fb0 --noverbose -a /opt/splash.png
StandardInput=tty
StandardOutput=tty
[Install]
WantedBy=sysinit.target
With /opt/splash.png
Might need to kill fbi when starting the kiosk.