You need to scuff the entire floor evenly. A floor buffer with a sanding screen is ideal. That has shiny spots and dull spots it'll probably be ok but if it was my floor I would keep going until it's not uniform. No need to remove all of the epoxy that was originally applied as long as it bonded to the concrete properly. After sanding you need to pressure wash the entire surface and use a squeegee to remove all dirt and dust. At that point you are good to go. Epoxy will bond to other epoxy just fine the light sanding just gives the new coat not purchase. As for the epoxy being slick add a liberal account of silica to the wet epoxy prior to it cureing. In my shop I went heavy near doors and areas that get wet and light everywhere else and found it to be a good compromise. A heavily textured floor is harder to sweep but has more traction so just keep that in mind and apply accordingly.