Check the flight from your airport for that weekend starting January 2021. Check once a week. See how they fluctuate. That will give you a good indication of when to book in 2022.
Once the rates are released for WDW they rarely fluctuate much. I say book as early as you can. Then watch for specials and discounts. It's usually easy to change.
Have you considered starting a WDW savings account right now? You're 21 months away. Put $80 a month in savings and you'll have $3,800 in May 2022. That's $20/wk. Twice a week when you are planning to hit Starbucks - don't.