How to Dim/Brighten a Group, Maintaining Relative Device Brightness Levels

[ccclapp]

Reading the zigbee2mqtt documentation I am trying to clarify a few quick items, any help would be great :-)

1. If one Groups lights together and wants them to be turned on/off at different brightness per light can those device specific brightness be set when creating the Group, or, do device specifics have to be done as a Scene (which has the Group inside it)? I n the Group or the Scene config documentation, I didn't see where the member device specifics e.g. brightness, get set.
2. Assuming the above Group (and Scene if needed), so multiple devices have individual preset brightness levels when the Group(/Scene?) is turned on/off...Next question becomes: If one wants to brighten/dim or change the brightness value of the Group/Scene as a whole, can this be done by sending a command to the Group/Scene to e.g. dim or be at some brightness value less than 100% (to the Group not changing the relative brightness levels of the separate devices). Example: A group w 10 lights. When it turns ON: 3 devices are at 30%, 3 are at 70% and 4 are at 100%. Now I want to DIM/set the Group to 50%. Thus 3 go to 15%, 3 go to 35% and 4 go to 50%, ie the Group Dims as a whole, maintaining the relative values of the devices within it. How is this done?
3. Finally, if I want to expose that Group Dimming (or set Group to brightness X) command to HomeAssistant, so I can have the equivalent of a dimmer in HA to turn the zigbee2mqtt Group on/off or dim/brighten how would i do that?

For context, I use ISY for Insteon lights and can use a physical keypad buttons that do on/off dim/brighten for a scene (group of lights). In Scene creation the relative On light values are preset for each device. When I dim via the KP button they all dim but keep their relative levels. I want to do this with zigbee (or zwave) and control from both a physical dimmer and virtually in HA, (and then including "Alexa set Light Scene Dinner to 80%")

Any guidance for any parts of this would be great.

Thanks