Theory about why Sozin waited 12 years to attack the air nomads

I've only read 3 of the comics, so idk if it's been clarified there already, and this is assuming he didn't know the comet was coming in 12 years.

Sozin was Roku's best friend, it would make sense that Roku would tell him about how the Avatar would completely end if he died in the Avatar state. I believe he might have been waiting for the next Avatar to know they're the Avatar and be able to take the Avatar state so he could have them killed and end it all, ensuring the fire nation would never know a real threat to his plans ever again.

Edit: turns out I just need to rewatch the series (oh no! Anyways) since it was clearly stated that Sozin knew the comet was coming and I just didn't remember it. Thanks for the responses, everyone!