I found Automagic as I was looking for an automation app which is able to use GPS speed as trigger. How your speed trigger is defined is ingenious

Coming to the point, I have quite a special problem. I have an Android head unit in my car, which uses a relatively new hardware (8 core processor, 2GB RAM, with Android 6). I wanted to use Automagic to define speed-dependent volume control on the device. Not a complicated flow: with GPS speed trigger, when speed is increased over the limit, action is “Set audio stream volume”, add one. The speed, where I raise volume, less the threshold, is the speed where in the opposite direction (lower volume) I set the trigger. 6x2 flows would do the job for me.
Now comes the problem: the head unit is not using any of the standard stream types! I tried all stream types available in your volume control action, none of them worked. I tried then your very useful “Set system setting” action. Amongst the available options I identified the bloody custom stream type, that they are using: it is called “av_volume”. As a test, I fixed a high value for this setting. As the speed was reached, Automagic changed the volume in the setting. I think you know already what comes now: since I am not using the API, the changed setting took effect only after a restart of the unit. But, set this aside for a moment, just go on thinking. The complete flow would be: speed trigger as written above --> “Init variable system setting” for “av_volume” --> +1 to the variable --> “Set system setting” according to the variable current value.
Coming back now to the API issue: the way out would be screen off and on. In practice: I set screen time out to 1sec --> wait 1sec --> screen on. What I would see in my car: the screen of my stereo turns off for a visible period of time, then turns back on, in the same time the volume changes. This is not nice: looks rather a bug, not a feature (despite that it is Linux, not Microsoft

Conclusion, and my request / question: do you see a chance to adjust the “Set audio volume” action, so that the not standard “av_volume” stream type becomes a selectable option? Or, do you maybe have an idea, how I could solve the problem?