Berlin late August What attractions i need book in advance?

I was there in February. The Reichstag was the only advanced booking / timed entry that was required.

The Gemäldegalerie was very busy on the Sunday that I went. There were at least 50-75 people waiting in line to buy tickets. You may want to get an advanced booking ticket. Or, do what I did. I simply stepped out of the ticket line, purchased an online ticket via my mobile and walked straight in.

I realize lines and wait times will be significantly different from February to August and your experenice may be very different from mine.

Confirm that Reichstag is best to visit with pre-booking.

Maybe also special events in August with limited access.

If you haven't booked the Reichstag yet, do that immediately. Very limited openings in August ( I see just three dates with one time slot each). https://visite.bundestag.de/BAPWeb/pages/createBookingRequest/viewBasicInformation.jsf?lang=en

Depending on your interests, a museum pass might work for you. But price out the things you want to see and determine if you still have to make time slot reservations (it has been awhile since my last visit).

I encountered long ticket-purchase lines on Museumsinseln in 2015, so if you want to go to any of those museums, I'd recommend getting online tickets. I don't know whether any need to be purchased much ahead of time, though.