Do you think it's a good idea to do preorders on this new hardware or should I list it on the site when they're ready to ship?
Your shipping history is confirmed to be of very high standards. So I would not mind preorders.
In fact it might even be a better option. It would allow me to be prepared for setting up the miners (calculating energy, space and cooling needed to run them). Few h-boards can be set up in no time, but even medium-large order takes some time to get online.
Maybe half stock as preorders and other half as "off the shelf" shipping when ready?