convert spline to round to straight

Its hard to understand exactly what you mean but if you go to vertex mode you can select all and convert to corner, smooth or bezier types. I think what you are referring to is smooth but it doesnt give you much control over the angle. What I find useful for a shape like this is make it all with 'corner' type and then select the 'fillet' option.

You should be able to right-click and bring up the quad menus. In that menu on the left should be options to convert the selected vertex to "corner". this will make the transitions of each segment from that vertex absolutely straight. If that's what you mean.

https://ibb.co/TqB10Lb
new update

this is the grill and my fisrst quite close resultt it seems .... the model is in gray . its the front bumper im creating ..... i though they have thos small round corners on thos shapes ..... the mesh ive create d in grey
https://ibb.co/QctkGrQ