\[ \left (y'(x)^2+1\right ) \left (a x+\arctan \left (y'(x)\right )\right )+y'(x)=0 \] ✓ Mathematica : cpu = 1.19628 (sec), leaf count = 58
DSolve[Derivative[1][y][x] + (a*x + ArcTan[Derivative[1][y][x]])*(1 + Derivative[1][y][x]^2) == 0,y[x],x]
\[\text {Solve}\left [\left \{y(x)=\frac {1}{a \left (K[1]^2+1\right )}+c_1,x=\frac {K[1]^2 (-\arctan (K[1]))-\arctan (K[1])-K[1]}{a \left (K[1]^2+1\right )}\right \},\{y(x),K[1]\}\right ]\] ✓ Maple : cpu = 0.043 (sec), leaf count = 30
dsolve((diff(y(x),x)^2+1)*(arctan(diff(y(x),x))+a*x)+diff(y(x),x)=0,y(x))
\[y \left (x \right ) = \int \tan \left (\operatorname {RootOf}\left (a \tan \left (\textit {\_Z} \right )^{2} x +\tan \left (\textit {\_Z} \right )^{2} \textit {\_Z} +a x +\tan \left (\textit {\_Z} \right )+\textit {\_Z} \right )\right )d x +c_{1}\]