Playstore>BxActions
Play around with the app, it works well. During the initial official release of the phone, when "professional" online reviewers had already had access and time using the S8, it was revealed that 3rd party apps designed to remap the BB were rendered useless or next to useless because of the fact that Samsung actually released an Over Air Update to kill those apps ASAP.
The 3rd party apps apparently overcame this issue as they were still available after the Over Air Update, but they had a SEVERE flaw: Even though you could remap the BB, it was a psuedo remap. Meaning, if you had your BB remapped to open Google Assistant, what would actually happen is this> 1.) Push BB 2.) Bixby Opens for 1/2 seconds 3.) Bixby is killed/closes 4.) Google Assistant opens. Sure it was good to have the option still, but the application/process was slouchy. It was really ugly seeing "Bixby" screen pop up for a nano second before opening the app/tool you set it to.
Well, at this point in time 11:30 PM PST California, the current "BxActions" app is damn nearly flawless. The issue I described above is ALMOST non existent. At least, I can claim I have NEVER seen Bixby try to spawn before opening my custom remap in day to day life.
There ARE other quirks I've experienced and am waiting for information online to surface from other users who might experience the same phenomena, but it's really so rare and slight that even I'm okay with (I'm a very stingy person).
Nothing will compare to a stock feature to handle this mess, but again, I HIGHLY recommend BxActions.