The questions are short but the answers would be long. Puppet manages all fine grained OS resources (files, dirs, repos, cronjobs, sudo declarations, firewall rules, etc) and you aggregate those resources into classes which are then pushed to different machines. The classes are parametrizable for the differences between systems.

If I was to write an idempotent script for each native resource I would finish in some years :-)

You chose whatever monitoring system you like the most.

For offline nodes you use whatever the level of criticity of your node justifies. This is something people struggle to understand: not every business needs 99.99% uptime. That said, I never had a downtime in Hetzner. On Digital ocean I had one short forced reboot in 4 years. YMMV so protect yourself as much as necessary.

Deploying on a different provider than Hetzner is the same as deploying on Hetzner except the part of launching the machine which is trivial to script - the added value is making the machine work and Ubuntu/Debian/RHEL are the same everywhere. You don't have vendor lock in with this.

If K8s works for you, enjoy it. Nobody is telling you to stop :-)