I also use the 15, and mine was at 87% around the 6-month mark. After enabling optimized charging in settings and not gaming while charging, it's been over a year and it's still holding at 85%. It might be because I only use fast charging, but if you've hit 80%, that's considered serious even without AppleCare. I'd recommend getting it checked at a certified service center. The replacement cost is around 150,000 KRW at official centers. Batteries usually last 1 to 2 years on average, so you haven't been treating it particularly well.