Having searched for ages trying to find an answer I tested a method I used ten years ago on an iPhone and it worked for me.
Do this, select your personal chosen sound file for messages and using a third party app to change it an .ogg file. Then install any third party app that adds it's own sound files to Samsungs stock ringtone (Notifications sounds) folder, I used Ring.com app simply because I have the doorbell. then upload your .ogg file to your phone, navigate to the audio or sounds folder of your chosen app, in my case :- my files>internal storage>ringtones>ring>audio>ring_app_alert_tones_oreo
I then chose a tone I wont use ie windchimes.ogg
go back to your downloaded tone and rename it windchimes.ogg (or whichever you choose) remember it is CASE SENSITIVE. Copy your newly named tone and paste it into the above (or your chosen apps) ringtone or sound directory.
Restart your phone, then go to your chosen messaging app (in my case Textra) select the newly named windchimes.ogg as your notification sound and you will now have your own preferred notification tone again for your messages.
You can probably now change any other .ogg file and select it for other preferred notifications in the same way, just remember to redo it if yor app is updated.
SoloUK
Do this, select your personal chosen sound file for messages and using a third party app to change it an .ogg file. Then install any third party app that adds it's own sound files to Samsungs stock ringtone (Notifications sounds) folder, I used Ring.com app simply because I have the doorbell. then upload your .ogg file to your phone, navigate to the audio or sounds folder of your chosen app, in my case :- my files>internal storage>ringtones>ring>audio>ring_app_alert_tones_oreo
I then chose a tone I wont use ie windchimes.ogg
go back to your downloaded tone and rename it windchimes.ogg (or whichever you choose) remember it is CASE SENSITIVE. Copy your newly named tone and paste it into the above (or your chosen apps) ringtone or sound directory.
Restart your phone, then go to your chosen messaging app (in my case Textra) select the newly named windchimes.ogg as your notification sound and you will now have your own preferred notification tone again for your messages.
You can probably now change any other .ogg file and select it for other preferred notifications in the same way, just remember to redo it if yor app is updated.
SoloUK