I'm not sure if it's just galaxy s20 but when I connect my car to my phone via Bluetooth and I play music via Google music an I have the volume on Max but I have to turn my cars volume all the way just for it to play loud on a good bose Audio system. Its not normal loud, my s10 didn't have this issue at all.
The way I troubleshoot the issue is I went to my radio in my car and the music 🎶 was so loud. So clearly this is a s20 issue. Not sure if it can be fixed via software update but this needs to be addressed.