My 100L with f2.8 produces a way too thin dof wide open.
well, you dont HAVE to shoot wide open. i think sporgons suggestion and mine is that you should understand what each focal length will behave like and what apertures are afforded to you given any given situation.
you could use the 17-40mm at 40mm and F4 and get usable shots. you could also use the 70-300 at 70mm and 5.6 and get usable shots. its all about understanding how each lens will behave in a given situation and what apertures are available to you given the look you are trying to achieve.
unfortunately this takes alot of time and shots to familiarize yourself with how your gear behaves in certain situations so you can confidently make decisions quickly during a shoot. i know when i can pull off F2 on my 35mm or when wider than F5.6 isnt an option on my 70-200mm. but i only know that through taking ALOT of shots where things didnt work out.
beyond that, considering the shots you posted, i would suggest you ditch shooting in sunny spots and seek out more flattering light. you are using the speedlights to fill well but it still looks flashy to me. softer, more diffused light is much more flattering for portrait work. you can still use a speedlight on camera with diffuser to pop a little catchlight in the eyes while shooting in shade and maintain a more natural light look.