Marriott Bonvoy stays have a funny habit of not posting properly. Then you must wait 10 days before being allowed to retro-claim. So any strategy that involves going somewhere else for a couple of days might just fail...

I would check out of the Marriott on Day 25 --> 50 nights with the double night promo. (or earlier if you already have some Bonvoy elite nights for 2022 and assuming that this is a paid stay) You could check back into the Marriott for a few more days on a separate reservation, or even as an OTA booking. Or of course, just move to a different hotel.

By the time you check into the Sheraton on April 1, your account should hopefully have been upgraded to Platinum.