Is there any way that y’all can think of to play the part of the emperor from warhammer in terms of creating the primarchs? I figure I’ll have to go for some kinda apothecary wizard, but how would I go about the genetic splicing and incubation?

You mean as an NPC?  If so, do whatever you want. NPC’s, especially your campaign’s villains don’t have to follow PC rules. If you want to gene splice just do it and call it magical experimentation or whatever you like

I don't know much about this particular character type, but if the focus is someone who creates minions I'd say take the Conjuration Wizard and reflavor your summons as being genetically spliced and created creatures.

Would it be possible to do something like that but make them permanent like using simulacrum and true polymorph?

Quote from Mr_Chip >>

Is there any way that y’all can think of to play the part of the emperor from warhammer in terms of creating the primarchs? I figure I’ll have to go for some kinda apothecary wizard, but how would I go about the genetic splicing and incubation?

Genie Warlock, and each Primarch is a simulacrum of you.

The Simulacrum is a bit unnecessary... with True Polymorph you can turn a door or a table into whatever creature you want. I suppose the Simulacrum at least ensures that the creature created will be loyal to you and have a specific personality, but it's still a big investment that isn't wholly necessary if you already have access to True Polymorph.