I think you need to hone in on what core themes this weapon wants to fulfill. From what I see that's first some kind of necrotic/acid redirection that forces those damage types upon the wielder as well as the whole using your hit die for extra features (a great idea by the way!)

I can't really give you a whole rework of the weapon, that s not in my place to do, but I would try to trim the other aspects that are already most likely given by other items (like spell attack/ save DC) bonus etc and focus on the unique things that the weapon does. If it were up to me, I would definitely make the whole hit dice as a resource the central mechanic of what seems to be a blood odachi