I just finished reading how GrapheneOS has specific hardware requirements so Qubes team suggests not to bother trying to build its iso into a qube VM
Thus the Qubes team has issued a “for hire” position for anyone who can Fork GrapheneOS to be supported on Qubes as a VM Template option
On the other hand, CalyxOS seems to have emulator support and documentation so I suspect it is far easier to get a qube VM up and running with CalyxOS than attempting to do so with an unsupported GrapheneOS
I have not tried this yet but I am planning to try, sometime this month or next