Make it a subclass. Limit the number of undead in your command at once by your level. Have it be a concentration spell where spellcasting through minions is limited. Have the limiting factor be your bonus action. At loss of concentration, undead remain on your side, but can't be commanded. This forces you to either spend an action to regain focus or makes your minions use normal attacks.
Introduce it as a hidden class that can only be unlocked after a specific questline in Bg. Reviving undead is not a well known discipline. It's also too strong for the early levels. If you arrive in Bg3 at level 5 and focus on that quest, you could have it available by level 6. Limit the strenght and amount of undead so you don't outdamage a martial classes' two weapon attacks at that time.
It's balanced if it's roughly one of their attacks in damage, since you still have a whole action left and commanding undead used your bonus action. This also means a necromancer will need an assortment of spells to do damage themselves as a caster. At this point, a preciser concept of the class is needed to divide their potential for melee, tanking and ranged damage. The first question to establish this is who can become a necromancer, and what abilities they retain.
Eldritch knights, warlocks and wizards all have a different angle. Warlocks wouldn't need to lose any of their prior features. Wizards maybe should switch out their spell list to an extend. Eldritch knights are currently underpowered enough that it depends on how much Larian is planning to torture them in the full release.
Along with haste being untransferable and some other minor tweaks (such as set spellslots for undead companions per short rest), it just needs to scale to level better.
