Love the swarm concept for animate dead. I wonder why I hadn't thought of it before.
I have a necromancer (true or adjacent) every campaign. Every campaign. Its always someone different, too.
Otherwise I try to limit any kind of summons to one creature, and try to make it interesting. I think my games tend towards transplanar shenanigans, so there is a fair bit of summoning and travel. I would expect any major spell casting class to be able to call some sort of otherworldly being. I also try to strongly encourage one pet or hench per player.
Familiars are housecat / crow / corn snake sized. They're spirits in animal form, so I have them with functionally the same stats. You have to do the ritual over again if they die, so there's a little financial sting. But, they do have a personality, and while loyal, you might wake up with poop in your shoes if you continue to send them to die. Or you forget the spell they taught you. Small, petty things to indicate their displeasure.
