I'm like you and highly irritated that they changed that. I'd rather have the custom SMS tones than the custom ring tones. I've heard though the forums that there are several apps that will do the custom text tones. I ain't found them. Like you, I've tried Textra, Mood and a few others. All of them don't have that feature. The reason is, custom text tones were assigned in the contacts app. Google or Samsung took that feature out. By removing the custom text tones, no one can assign custom tones. Now people will argue saying that you can assign tones to the thread and that's just as good. No, it's not. By signing them to the thread, you can never delete that thread. If you do, no more customization for that thread. So I have to maintain every thread I've had to maintain any customization. Sorry, too OCD for that. Additionally, it don't work for group messages. You get a tone for THAT thread not that person in the group.
Yeah. Sore subject for me. I'll get down off my soap box now. LOL