Ok, so I've found that the main issue is that all volume control in Kodi only changes the volume for Kodi stuff, like the player etc. What I had to do is to edit the includes.xml file for the skin volume buttons.
Instead of "<onclick>VolumeUp</onclick>"
I changed it to "<onclick>System.Exec(/storage/scripts/volup.sh)</onclick>"
This allowed me to write a tiny script to change the overall system volume by a percentage (ie 5%) and leave the Kodi volume alone at 100%. This way I can change the volume of any and all audio leaving the RPi4 audio jack or addon audio card.
I used these in my volume script:
otherwise you can setup some scripts and button mapping to use:
pactl set-sink-volume <pa-device-name> +5%
pactl set-sink-volume <pa-device-name> -5%
ThanX Irusak for that idea! I couldn't make use of the pulseaudio output setting in Kodi, as my bluetooth is a separate straight through command line script for pulseaudio to auto route a connected bluetooth device (source) to play audio out of the audio jack (sink).
This doesn't give me volume control from my phone, but at least I have a universal consistent volume control for the system. That is very necessary and good enough for me!
On a side not Kodi remembers the volume of the system on reboot, so I included a line in the autostart.sh to set the volume very low on each start-up.
I'll check into the Kodi remote app later, but would rather not be switching apps to change volume.
