You're not along in missing individualized notification settings. It sounds like this was a feature that was nixed with the Oreo update. Unfortunately, there's not a way to get it back unless Samsung adds it back in when they roll out another software update.
Just wanted to add on to what tmo_amanda had to say which is many users across various forums have turned to alternative messaging apps to add these custom notifications. Textra seems to be the popular one although it is not something I personally use since my phone stays on vibrate.
Just checking in to see if you have any other questions. Please come back and let us know. Thanks.