Yeah I looked at tutorial. Port 81 is only for management (NPM admin gui). Then you have your traffic ports for proxy services. Those would be 80 and 443 normally. You would need to expose those ports to the Internet if you want to access NPM/proxy your service. Port 81 shouldn’t be exposed on your public interface make sure it isn’t or at least have firewall rule to allow only local network (ideally management network/vlan)
Luks full disk encryption and encrypted offsite backups. This protects from most common smash and grab scenario.
I had issues where system upgrades would loose encryption keys and full restore from backup was my only option. Nextcloud have issues with encryption, some features are not available if you enable it (don’t remember which ones now).
Generally speaking if someone has physical access to your system you’re screwed. There are many ways that physical access can be used to get access to your data including denying you access to your data.