I’m still pretty new myself, so if there is a better answer, I apologize.
by slowing the sutter speed what you would do is underexpose the image, so that won’t give you the slow motion effect you are seeking. What I would do is shoot at normal speed and then when you edit your project change the speed of the clip. I use Final Cut Pro and you do is highlight the clip and change the duration. I’m not sure what editing system you are using, and quite frankly I don’t know any other system besides FCP.
Best of luck.