It doesn't seem worth it to spend so many diamonds to finish Sweep Thr Board. The last part is only..

Many players finish without playing diamonds by studying and/or practicing their Sweep the Board scenes in the regular gameplay beforehand. Try asking Google for June's Journey scene with 3 or 4 of the objects in it to find out which volume-chapter-scene to study. There are many sites that can also help. Facebook group https://www.facebook.com/groups/sweeptheboardoriginal has dedicated volunteers that can help you find the scenes, too.

Before Sweep the Board, decorations were offered for purchase at the cost of much more diamonds. In comparison, players say Sweep the Board is a much better discount if you can get near the end of the run.

I used to play sweep the board all the time, however since I stopped buying items for the game I have noticed there is no way to finish the scenes in 19 seconds without using diamonds no matter how much you study the scenes before hand. So I play as far as I can then quit before using diamonds.