Today, after having to wait for my new subnet over the weekend, I was continuing my journey to ESXi VMs on a Hetzner root server.
When installing ESXi on a Hetzner Root Server, tell tech support what you are trying to do and they will be helpful and your job will be much easier.
To get the machine running with ESXi (which does not support software RAIDs, so a Hardware RAID Controller is necessary) I added those options to the machine.
My first attempt to follow the docs did not work out. I did get the KVM console attached by hetzner's techs and was able to boot from the provided ISO, but ESXi 6.5 does not support the built-in adaptec RAID controller by its own – so I got stuck there, not being able to install ESXi to the machine.
So I sent a support request to hetzner's tech team, asking for advice. Even on a Saturday morning, within a couple of minutes, tech support replied. Preconfiguration was not available anymore, but "without any warranty" they can boot the machine with an ISO that already contains the driver.