Disable password authentication on SSH
Enable firewall and block all ports you're not using(most firewalls do this by default)
Switch to a LTS kernel(not security related, but it keeps things going smooth... Technically it is safer since it gets updated less often so it is a bit more battle tested? Never investigated whenever a LTS kernel is safer than a standard one)
Use Caddy to proxy to services instead of directly exposing them out
HTTPS for web stuff(Caddy does it automatically)