The only solution I have found has to do with a third party older version app call Contacts+. This does not force you to keep past threads of text messages in order to assign a text tone. I did have to modify Contacts+ APK so that it would not automatically update to a new version where they removed the text feature. I am not a programmer and this is something called re-signing the app. And there is an app called apk signer you use I think after Apk Exrractor extracting the apk. So I did that and it works about 95% of the time. You can see if a text message send doesn't work when after you send it it doesn't show up in sent or it doesn't say message sent. So if I write a long text message I usually do a select all and copy before I send it. The version of Contacts+ that I downloaded from the apkmirror extracted and re-signed it is version 5.118.0
I forgot to add I needed to ensure there was a directory called Media and under that a subdirectory called Notifications where you put your sound files for the text tones in the sdcard directory even if you do not have an SD card. By the way I am running on Verizon a Samsung S8.