Why have startup scripts when you can run your very own images? `Installimage` (https://docs.hetzner.com/robot/dedicated-server/operating-sy...) allows you to install and run your own custom images, so run any bootstrapping once, create image from the results and then install that on the machines.

If you really really need startup scripts, it'll be trivial to add it to your own image. Packer (and similar tooling) makes it trivial to create your own images as well.