Until the disk is attached to dom0, the odds dom0 to get compromised from above scenario are very low. Xen exploits are needed for this, mostly. So, never attach that disk to anything else in Qubes, at least. Just as you created dedicate bitcoin-qube, decide that disk to be bitcoin-dedicated too.
Whose security? dom0? Not. Nothing else left, since it would be decrypted in bitcoin-qube, and wouldn’t be used anywhere else, but even if you’d want to use it anywhere else AppVMs and dispVM are considered to be already compromised by default in Qubes.
You’d want to encrypt disk attached to dom0 from a physical access, and once an adversary in dom0, it’s already Game Over.
attach only the partition, not the disk → the bitcoin-qube will not see the partition table of the disk and can’t compromise it
encrypt the partition → dom0 can’t see the data contents of the partition, however it can still see some partition information/metadata so I guess this won’t improve security a lot, and besides, dom0 shouldn’t touch the data anyway even if it’s not encrypted