All the villagers in my second town are original, both their clothing and furniture as well as catch phrase and greeting. I have that town since December last year and everybody is still original.
Here's what I usually do:
1. 'Say yes' (or no) to every request but don't actually do them except for fruit and signatures. Also, I agree to home visits but I don't take any of their furniture.
2. Never give them bugs or fishes, they're just going to display them on top of a table or somewhere inside their houses.
3. Be wary of delivering packages because they're mostly clothes and again, the possibility that they'll put them inside in exchange for the original furniture.
4. Fill the Re-Tail with shells or anything. Replace those clothes from "Anytown" with custom dresses or umbrellas in Able Sisters
5. Don't make them buy anything from your pocket (especially fossils!!!)
6. I always say no or "you're fine" something like that, if they ask to change catch phrases and greetings.
7. Avoid being idle in a specific spot where villagers walk around (laying your paths, been afk or doing nothing). They might exchange clothing during convos. Either open your train station or often save/quit or just save if you think you're gonna stay in one area for too long (just to be safe in case something happens).
I don't think it affects the friendship if you don't always give what they want except that it probably takes a bit longer to establish a high friendship with them. I haven't noticed the difference, to be honest. If you really want the pictures really quick, give them perfect fruit. They won't move out just because of that. Villager asking to move is always random, no matter what.
Reporting to Isabelle about their language makes them say "uh-huh" and makes them change to plain white if you report about their clothing.
