Juryaan, I’m at the same place you are… I have only been able to find brass slotted screws in my local area and they’re too soft.  I did run a practice bead with one, but mostly ended up sawing it to depth and rounding over with 80 grit sandpaper.  Worked okay, but not as efficient as letting the screw do the work.

For folks in the US, where have you found good quality steel slotted screws?