I moved away from systemd to xdg autostart instead which executes the script as the correct user allowing permissions to be set by default

/etc/xdg/autostart

[Desktop Entry]
Type=Application
Exec=/home/EB/EBSYNC/app/chromium.sh
Hidden=false
NoDisplay=false
X-GNOME-Autostart-enabled=true
Name=Chromium Kiosk
Comment=Launch Chromium in kiosk mode