Firewall and deciding on an entry point for system administration is a big consideration.

Generating a strong unique password helps immensely. A password manager can help with this.

If this is hosting services reducing open ports with something like Nginx Proxy Manager or equivalent. Tailscale and equivalent(wire guard, wireguard-easy, headscale, net bird, and net maker) are also options.

Getting https right. It's not such a big deal if all the services are internal. However, it's not hard to create an internal certificate authority and create certs for services.

If you have server on a VPS. Firewall is again your primary defense. However, if you expose something like ssh fail2ban can help ban ips that make repeated attempts to login to your system. This isn't some drop in replacement for proper ssh configuration. You should be using key login and secure your ssh configuration away from password logins.

It also helps if you are using something like a proxy for services to setup a filter list. NPM for example allows you to outright deny connection attempts from specific IP ranges. Or just deny everything and allow specific public IPs.

Also, if you are using something like proxmox. Remember to configure your services for least privileges. Basically the idea being just giving a service what it needs to operate and no more. This can encompass service user/group names for file access ect.

All these steps add up to pretty good security if you constantly assess.

Even basic steps in here like turning on the firewall and only opening ports your services need help immensely.