One scope, Skywatcher Eqm35-pro goto, ASI120 guide, D7200 imaging.
Think of this as a project, rather than an ideal solution.
Is it possible to install Astroberry on each Rpi4 and have each child take on their own part of the task, such as one for imaging and guide camera, one for mount control and one for PA and plate solving. With overall control from one PC in the house.
Can Astroberry be configured in this way? (I'm pretty sure ekos/indi etc can), or would be a case of Astroberry on one Pi, and indi servers on the others?
Well, yes, no, partially (I believe). But what is the point in doing it like you suggested?
You can have each raspi attach physically to a different device and start an indi server on it. But this is with-out UI. Then you could (in principle) have another indi server / indi hub, that then provisions all these servers to the UI (kstars/ekos), which then controls the rig. (Or you could attach the imaging software directly to all these different, remote indi servers). But then there`s only one instance controlling the imaging rig.
I would supply a Raspi to each child and then connect the imigang rig to it in turns. So each child is responsible for its own instance of kstars/ekos, configuring and controlling the imaging with it.