I recommend the mod NPC Control, which has a function to stop the game from randomly generating sims, and using what's already in the world to populate everything. Using this, and flooding the game with downloaded sims, you can still have a pretty full world.
I don't use mods or CC...too many bad experiences over the last 20 years with mods/CC breaking the game and causing the loss of entire worlds when you have to repair/re-install the game. I really wish the devs would cook some more customization options into the gameplay section for TS4. That way I know if something goes wrong - it's not because I have 15 mods running and 3000 pieces of CC mucking everything up.
If you haven't tried this yet, you can also make your own townies. Plop a household of sims you want to be not-in-world npcs into a lot, save, go to manage households, & immediately evict 'em; they'll be categorized as 'not in the world'. Un-heart them, & you've got a new npc-townie household. Rinse and repeat. Save when done.
I used to download 8-sim households of townies that various simmers made for the purpose. However, the game never assigned those sims jobs as bartenders, etc., so I shifted to one-sim households only. Can't remember if that worked or not! Now I don't bother with the townies at all; just clear out (delete) the oldest ones out periodically.
However, this might suit your needs! Hope so, anyway.
