Sorry, but the volume of Maps, Apple Music, SIRI, everything is a global setting. You can ask SIRI the volume and the response will be in volume percentage. I even have a shortcut that raises my volume to 90% when my iPhone connects to my car’s BT, and back to 55% when connected to my home WiFi.
I was finding days when both the Map voice navigation and Music were at the same relative volume, but that is at lower speeds. I wanted to see if my car’s adaptive volume setting (based on speed) was the culprit, but I haven’t had time.
PS: if you search in settings, Apple left behind the Navigation Voice Volume, but it can no longer take you to that setting.