Dark Old Demon
He could make simpler more effective changes instead of these more radical ones. The tail is fine since it makes up for one of his weaknesses but instead of something as abstract as the T-virus, he could simply change the properties of his own body like larger muscles or hardened skin. It shouldn’t be that expensive as the T-virus even if the effects aren’t as good either.
Why doesn't he just make a potion to quickly make his body grow and age into its peak natural form? Then he can easily and effectively hunt more prey. He would then have more souls at his disposal in a shorter time and have less to worry about in terms of predators after all even if he is a mix blood with less potential than his fellow demons, hes still a demon and so he is stronger than most low tiered beasts. At least from the description given. Then he can use the many more souls to make more radical changes to himself like his body or bloodline.