For my part, I would create the assembly with the 2 flats and the pin without the hole so that all the parts are constrained. Then the would create a sketch in the assembly using a plane of the tapered pin and convert the pin in the sketch and draw a line in the middle of the sketch to cut it in 2 and adjust it, then I would do a material removal with revolution and propagate the function to the parts.
In the idea, @manu67's solution is the right one so as not to bother too much.
Make the pin hole and create the pin "in context" of an assembly.
The only flaw is that the pin is then dependent on the assembly. This may not be a problem. Otherwise, break the references to the assembly or create a new pin identical to the one studied in the assembly and replace it.
Thank you for your solutions another (draw the sketch below):
Insert - Sketch Draw a circle. Tools - Sketch Entity - Circle Pierce the material. Insertion - Material removal - Extrusion Choose the surface on the part. Set the End Condition to One-Eyed. Adjust the Depth. Click Draft On/Off and enter the angle measurement.