I'm on a 14" M1 Max from 12/2021 and my AC+ expires in 2025. I'm pretty sure you can buy it right before your warranty expires to extend it. But maybe you bought 1 year of AC+ and can't extend it after?
This was my first AC+ purchase.
Also my battery has been at 79% Service Battery health for about a year. I forgot to use AlDente to limit the battery to 80% but do it now and it hasn't gone down.
edit: Jesus I just realized 2025 is next year. Totally thought I had like 3 years left.. nevermind..
That's an Apple hardware classic. Sorry to report that your GPU might be shot. (Hopefully it was just a one-time fault, though.)