Why not make a separate volume level for messages and ringer? It would be very convenient.
Message and ringer volume.
Perhaps there is such a function, but I do not know about it.
Let’s take it a step further and have separate volume levels for contact groups/contacts; Ie Work volume 5, VIP volume 10, Family volume 10. Simultaneous same for communication apps; WhatsApp volume 5, WeChat volume 8. While I can understand that this could add confusion to many low level users, for advance and business users this could be a great feature.
It is very unlikely that Apple will do this.
However, they have provided a mechanism for you to do it yourself...
You can create the same effect, by making your own ringtones and message tones.
When you record the tones, you could adjust their volume levels, to suit.
You can even have per-sender message tones (with different volumes), to distinguish between more and less important senders.
I use the Fission app to produce my tones... many such apps are available.