I did just check again and it does look like April 10th doesn't have Standard award availability now (I may have missed that night earlier), but every other night still does. I have just made a dummy reservation for 04/12-19/09. I picked the 4th option down (the first three were Pointsaver awards requiring the upgrades you mention):

"Guest room, 1 King or 2 Double, Garden view, Garden Houses Room details
Standard Hotel Reward 130,000 points for stay"

And using Marriott's Alternative Date Finder, every day in April except the 10th has base Standard award availability.