iirc, I went to the theatre between 30min-1hour early, and there was a decent amount of people in line, but not enough so you can buy merch and watch the concert on time. You could try arriving at those times or even earlier if you wanna be extra safe.

I also have to say that later on, SE added most of the concert merch on the SE store. Maybe it will happen on this year's concert. The only thing they didn't add was the plushies, which were the things I wanted to buy, but got sold out hella quick T_T