Don't Wait if You Absolutely Need a New Mac
Is your current Mac broken, or does it take too long to get your work done?
If you cannot get your work done, complete your school work or otherwise use your Mac, it may not be worth waiting for the new model.
While there is a very good chance that the new iMac is coming soon, Apple may pull a surprise and not start selling it until later in 2020. Factor your current workload and commitments into your decision.