The first thing everyone should do before connecting to the Internet is to set up a proper firewall. Since Debian 10 (Buster), the iptables framework is replaced by the nftables framework.
To make it easier to get started, here is a short guide on how to get your firewall up and running in no time.
The problem is not that’s to easy for you to log in, the problem is that everyone else can try it too. If there are no additional mechanisms in place, such as fail2ban, an attacker will have endless time to try to guess your password or even worse the root password.
First lets start with the key generation for your client
ssh-keygen -a 100 -t ed25519 -f ~/.ssh/example.com_id_ed25519