You are correct. The Ringer volume and the text tone alert volume are the same. There is no way to set the two of them differently. Have you tried a different text tone? Are you looking at the iPhone when you receive the tone? If possible, then it may be that you are seeing the effects of Attention Awareness Features. This will reduce the volume of some alerts if the iPhone determines that you are looking at the screen. Go to Settings>Face ID & Passcode>Attention Awareness Features, and turn that off. Then see how your text notifications sound.