There are no slip-ons for the 2017+ 650. The whole exhaust is one single component, so short of chopping the OEM exhaust in half you can't install a slip-on. And no manufacturer would sell you a custom-designed one.
Full exhausts are the way to go. If you want to go the cheap route, I'd recommend Danmoto. As far as the premium options, I went with the
Yoshimura Alpha Full Exhuast. Works fine without a tune and sounds great. Is it a sensible upgrade? Well... not economically at least. But I don't regret installing it.