It's designable, but it's going to require some swings.
WOTC is (understandably) avoidant of summons, which makes sense. Even a single extra-NPC is a full new set of health to track, attack rolls to figure out, damage to deal out, etc. It can significantly increase the time a round takes.
But...it seems to me, embracing 4E-style minions could fix the problem. Let the necromancer either summon a swarm (single entity, just like we see in 5E) or a number of 1-hp minions that use the necromancer's spell-attack, with set-damage (no dice roll needed).
This is a steep-departure from typical 5E, but with a new leadership team in charge, maybe now is the time to try some out-of-left-field adjustments?