I don't know what sort of schedule they use for ticket release, but I've never heard that the Theatre-Museum is a sight for which you need to buy tickets way in advance. (The house in Port Lligat is a different matter.) I know the Theatre-Museum does get some tour groups, and those seem to prefer fairly early entry times. I suppose if your schedule is such that you really, really need to go early in the morning, that would call for a bit more diligence in checking the website. In May 2019 I was able to get a rather early entry ticket by making the purchase just two days ahead of time; there may have been some luck involved, and July is likely to be busier.
In case you are not aware, the Theatre-Museum ticket also covers entry to the nearby jewelry collection, which is quite fabulous. I was told ticket holders can visit the jewelry collection either before or after their ticketed time, so that will give you a bit of flexibility. I think 30 minutes would be sufficient time for anyone (even photographers--but that probably will not go well unless you're a pro) to see the jewelry collection, which isn't exceptionally large.
I spent considerably more time in the Theatre-Museum than I had expected--over two hours for sure. I am not a fan of surrealism, so I had expected a quicker visit, but the place isn't small. If a tour group shows up while you're there, it may slow you down. I found the tour people didn't go upstairs, so I hid out up there until they left.