Is it possible to install all boot partitions/metadata on a removable disk?

The removable disk would thus be a “boot key” for the machine. The hard drive would only have encrypted data, with no way of knowing that Qubes is installed in it.

Is this possible at all or is it asking too much? I’ve done some over-convoluted Linux installs before but with this one my mind is blank

I think there is existing guide for this, which you could find by
searching Forum.
You can either do this at install stage, or do a standard install and
then move the boot partition to an external drive.
There’s nothing Qubes specific here. You can find many guides online.