If it’s a Debian system, “Create user with sudo privileges” and “Disable root login” can be done during initial setup. Just leave the root password blank and it’ll disable the root user and grant sudo permission to the regular user you create.
Create a separate management VLAN and use it for all your infra (web UIs of all your networking hardware, Proxmox, SSH for servers, etc).
For unattended upgrades, ensure the auto updaters are properly configured so they’re used ONLY for bug and security fixes, nor for minor or major releases! Debian unattended-upgrades has good settings out-of-the-box but you may want to add any custom repos you’re using. Make sure you have an email relay server configured in the Exim config, as it uses apt-listchanges to email the changelogs to you.
But above all, press the power button to turn it off and then never turn it on again. 100% unhackable.
+1
Use unattended updates ONLY for bug and security fixes, nor for minor or major releases. Ensure you configure your auto-updaters properly!
Debian
unattended-upgradesonly upgrades packages from the main and security repos by default, so it should be fine since no major updates are performed within a particular Debian version.