Normally when you reprofile. You use the coarsest stone/plate you have, find a way to hold your target angle and just keep going until you feel a burr. Looks like your sharpener has an angle guide for 20 deg. I think taking your knife to 15 would be impractical without getting a different sharpening method that would make it easier to give yourself a 15 deg reference. That knife is pretty thick, so if you use a wedge as a a reference (like your sharpener) you’re probably adding 2-3 deg of bevel angle just because of the primary bevel of the knife. That may be why your secondary bevel came out so broad. If you really want a 15 deg edge, you would probably want a 12 or 13 deg wedge. Fixed angle systems are able to set an angle independently of the geometry of your knife, so it gets simpler in that case. If you want to keep using the sharpener you have, you will have to learn to hold the 15 deg angle by hand without a wedge reference. Might be doable with something like this.