::shrug::
If you're really going to automate the process, there's quite a bit more to it.
I've never been inside any of the large industrial cylinder-filling operations, but from what I understand, they have lots of whips and fill through an orifice that limits the fill rate, and regulate the supply pressure to be equal to their desired fill pressure. No solenoid valves required.
Much of the labor is in attaching and detaching the whips, and some operations try to streamline that by using lever-operated yokes (for medical valves) or oversized handwheels (for CGA). I don't know if there's an equivalent for scuba valves.