You're asking about two different things, MQM and MQD waiver.

For the signup bonus MQM, it's been a while since I dealt with that, but I believe they will post in the year you hit the milestone, but that is not an issue for 2021/2022 because everything rolls over, and if you plan on maintaining status in the future, it's still not an issue because of rollover MQM, so I think you're fine there.

On spend and how it counts towards earning MQD waiver and also spend towards MQM boosts if you plan on earning those, they look at individual transaction dates, so while you may want to go ahead and apply for the card now, you want to wait until January 1 to start spending for your MQD waiver. So, if there are bills you can wait until January 1 to pay, do that. There will be a counter on your AMEX page and on the Delta website for spend, the one difference if you have more than one Delta card being the AMEX site has a counter for each card (which matters for MQM boosts) and Delta.com shows all spending combined (which is what matters for the MQD waiver).