Mod to stop playable sims being used by the game as food vendors

I think the Sim Spawn Overhaul mod has a function that only pulls "not in world" Sims for NPC roles. Which I think is the simplest solution to your problem. I know the mod consists of multiple parts, so you may only need one. I haven't had a chance to try the mod myself though.
There are other mods that can do this, but those require more actions to set this up.