I don’t use mods. Sometimes they would cull townies I had. But, some time ago they made it where if one of your played sims had a strong relationship with another sim (very high in friendship, enemy, or love ), that it would keep them from being culled. I’m not sure if it is still that way, but I assume it is. But, if you don’t play and keep up those relationships, they will dwindle over time. Also, if I really want someone saved I will put them in a house, apartment etc. so far I haven’t had people I have put in homes or apartments culled. Also, if you go in and make over a townie, I found they are more likely to stay around. I have lost mail people, service people, and waitresses. If you particularly like some of those people you might want to build up and keep a high relationship with them. I sometimes will cull people myself, but be careful, you may get clumsy waitresses or not so good cooks, etc.
I also think a strong relationship with another sim, especially an active family matter. I read the information about the culling system a long time ago.
I didn't have the auto-culling before because I keep almost sims in "My household". Even an NPC who doesn't have any friends is still safe. But maybe it is because my population has reached the limit already.
When I visit that save next time, I will try your suggested method such as moving them into the house. I may try to move Erwin Pries (Strangerville NPC in the hidden household) to some lot and check if this can help him not be culled.
About restaurants, I usually make my playable sims own the lot, hire and improve their skills to avoid low-quality service. The auto-assigned employees mostly start with low skills and do badly like you said.
