I prefer to use the built-in volume options of Kodi to change the volume of all media. This way I only have to use 1 remote and it also works with the Kore app on my phone.
This always works except when I watch media with Dolby Digital AC3 as the audio stream. Then the Pi4 does pass through and the audio is out or reach for volume manipulation. When its AAC or DTS or any other multi channel format transcoding kicks in and the volume can be changed from within Kodi.
As a workaround I would like to see an option to always transcode all formats to AC3 even when the original is already AC3. Then I won't ever again have to grab the receiver's remote when I'm too lazy and comfortable on the couch. 😎