Gos4dwin There will be a trace left on the installer device (the recommended android device)
Yes
Gos4dwin Also a trace left on the new graphene device
This is unclear to me, but there is nothing to lose by doing a factory reset.
Gos4dwin A factory reset of both devices will delete any trace of the other ever being connected to it? (Including a factory reset of the new graphene device)
Yes, per the reply from GrapheneOS above.