I am not a specialist on this, but I have experimented a bit. I know what is needed is a feature not available in LibreCAD - Spline with connection angle. Is there a workaround?
Tools - Curve - Spline. Set the first control point at the one end of the straight sides, the second at an enlongation of it, the third at an enlongation of the arc, the fourth at the end of the arc.
When you click on the resulting spline, you will see it marked and the control points shown. You can move them now to influence the spline, but they have to stay on the enlongations (blue in my example) for not to get bumps.
Choose spline degree 2 or 3. I don't know the difference.
Good luck.