Firewall should block everything by default, and you start allowing incoming and outgoing connections when you need them.

Disable passwords and root access in ssh daemon.

Use fail2ban or something similar to block bots failing to log-in.

Use random long passwords for everything (eg: like databases). And put then in a password manager. If you can remember the database password, it’s not strong enough. If you can remember the admin password for a public web service, it’s weak.

Don’t repeat the passwords. Everything should have its own random long password.

.env files and files with secrets should be readable only by its service user. Chmod them to 400.

Monitor logs to see if something funny is happening.