There really isn’t a secure OS everything has some sort of way in. I recently installed Ubuntu and encrypted it and thought hey that’s secure. I popped in my parted Magic disc after reboot and saw an encrypted volume and the boot loader that had read and write enabled.
They don’t give you the option of encrypting your boot loader so anyone can tamper with the boot settings so this is the biggest backdoor on Linux!
Alpine Linux is super small if you want to focus on security lower your foot print, encrypt boot loader and file system first before setting up firewall