This problem could be fixed, in principle:

Merlin'ss Interdict preventss powerful sspells from passing through ssuch a device, ssince it iss not truly alive. Dark Wizardss who think to return thuss are weaker, eassily disspatched.

You need two cooperating Dark Wizards. (I know, much easier said than done.) They share knowledge of all spells. One of them makes a horcrux, the other teaches him all the spells again. Then the other makes a horcrux, and the former teaches him.

Or maybe you just need one Dark Wizard, and one loyal servant, willing to make the Unbreakable Vow. Servant makes a vow to teach the reborn Dark Wizard all his spells, but never use them for himself or tell them to anyone else.