I managed to fix it with these options I got from reddit:
Do this: Pull down the notification tray all the way down. Click on Smart View. Turn off.
If you haven't set it up or turned it on, don't. Just back out and do this:
Settings > Connected Devices > Turn off Music Share. This won't help with the notification, I just now saw it existed and was on by default. Wtf Samsung? I don't want random people to hear some of the audio I listen to.
Now do this: Settings > Google > Devices & Sharing > Cast Options - Turn this off. If you have more than one Google account on your phone, go back two screens, switch accounts, repeat on every account.
That should fix your particular issue.
Go through the other settings under Devices & Sharing and and look at those just for good measure. I turned them all off only to later find out Google Pay uses one of them, I can't remember which.