Extended warranty is only worth doing if it's a genuine Audi one and not an independent company as most of them aren't worth the paper they're written on.
Your new allroad sounds lovely 🙂
Welcome to the forum you'll find the members on here are a friendly and helpful bunch 🙂
I'd recommend you speak to autologics or Audi retrofits as members have used them for retrofits and been happy with the results
If it happened 3 years ago and not since then I wouldn't be worrying about it. As a rule of thumb no 4x4 should be driven in constant 4 wheel drive on tarmac as it can prematurely wear out the diff, gearbox, driveshafts etc.