Oh right, crap, didn't even think of putting five moves there. So I guess you kinda gate out the content too early with too hard of a level if you do have some additional types of blocks too. Getting introduced to different types between similar levels (before going back to basic but difficult) would also greatly alleviate the monotony too. But yeah, getting difficulty right (in puzzle games especially) is hard. And even harder without external testers.
Puzzle games like this should always have some leeway to be able to skip for example one level to balance out the differences in players. I've tried even going as far as giving one level beat basically ability to skip four levels. The game (my last GitHub Game Off entry) had a star system and getting the max three stars and a completion would then unlock the next 3+1 levels. Noticed a pretty good amount of people beating the whole game of about 20 levels without really abusing the skips and getting comments about some levels they had to skip.
The play button is not needed! The functionality itself might be but the button isn't. The first click on the number keys could do the same thing really. And even the stop button could be kinda built in to happen when you just reorder the letter. That might be a bit annoying without warning though since it resets the puzzle.
edit: No need to give out the solution. Would probably find it easily enough knowing that could do 5:3.