How are you planning to get your luggage to the MK? Perhaps you could Uber or Lyft to the Contemporary and store your bags there? It's a short walk to MK.
I don't think you can store bags at CR unless you are staying at CR.
Generally, they don't want guests to have luggage on the (resort to park) busses, unless your luggage is day bag small.
As someone else said, if you have a car, you can store your luggage in the trunk of your car, if you'll have one. You just have to keep in mind that MOST OF THE YEAR- your car will get VERY HOT. Clothes are fine, but items like cosmetics, meds, food - esp chocolate! can get too hot in a hurry.
Your hotel, even the values- will store your luggage for you, and they even have a fridge you can use. Your stuff will stay in AC, too.
I suppose another potential option is to ship some of your items, but that gets expensive.
When we do split stays, we usually take care of our transfers in the morning of the day we check out to avoid this situation. We've never felt the need to waste a whole day either. It you don't unpack very much, then transfers are easy. Mostly, we just unpack toiletries (for 1st part of trip).
SO one potential solution is to change your hotel reservations: Add a night to your WDW reservation, and reduce your Universal reservation by a day, then do the transfer early in the morning. WDW will usually let you add a day no problem. Universal is slightly less flexible in how they do such changes, but if a room is available that night, they will let you book it.