Be careful, I played the waiting game last year and by the time I started buying, some merch had already sold out. The Weeknd XS T-Shirt eludes me to this day haha
I'd say pick your must-haves and grab those while stock is ample, then wait for the lower tier merch.
Agree - for clothing, just buy it now. That will sell out, especially all the Medium-Large sizes.
For the souvenirs, cups, coasters, mugs, etc. go on discounts the last week.
Not too sure when discounts will occur, last year it started ridiculously early since they had supply chain issues so they ended up getting a ridiculous amount of merchandise late in the season.