I have slight doubts about the correct end game.
Don’t want to give too much away so will talk about the straight lines in terms of the order of where they start in relation to the top of the puzzle.
The one that starts 3rd from the top could work with 4, 6 or 7 cells. I favour the latter.
The one that starts lowest in the grid could work with 5 or 7 (minus 1 crossing making 6) cells. Again I favour the latter.
I need 12 cells to get to 39. Is it 7 and 5 or 6 and 6?