By default your OS is secure. You only have to think about what you expose and how can it be broken in. Disable SSH password authentication. Don’t run software that is provided by hobbyists who have no enough security expertise (i. e. random github projects with 1 or 2 contributors and any software that recommends install method curl <something> | sudo bash). Read how to harden the services you run, if it is not described in the documentation — avoid such services. Ensure that services you installed are not running under root. Better use containerized software, but don’t run anything as root even inside containers. Whenever possible, prefer software from your distro official repos because maintainers likely take care about safe setup even if upstream developers don’t. Automate installing security updates at the day they released.

What doesn’t help:

• Security through obscurity. Changing SSH port etc. Anyone can scan open ports and find where SSH is listening.
• Antivirus. It is simply unable to detect each of numerous malicious scripts that appears every day. It just eats your system resources.The best it can do is to detect that your host is compromised, but not prevent this. It is not security, just marketing.
• Making different rules for public internet and DMZ. Consider there’s no DMZ. Assume that your host can be accessed by crackers from anywhere.