Why does it need to be public-facing? There may be solutions that don’t require exposing it to billions of people.
Security is always about layers. The more independent layers there are, the fewer the chances someone will break through all of them. There is no one technology that will make your hosting reasonably secure, it’s the combination of multiple.
You’ve already mentioned software ran inside an unprivileged sandbox.
There’s also:
Sandbox ran unprivileged inside a VM
VM ran inside unprivileged sandbox
Firewall only allowing applications to open certain ports
Server running all of that hosted by someone else on their network with their own abstractions