Hi Michael,
Not sure I have code available. At the moment, we don't expose the right properties on the symbol objects to modify the Arcade expression (deeper symbology API is coming in the next couple of releases too - you can imagine a lot of this is interrelated).
In the example I've seen, this Arcade expression is created in ArcGIS Pro to drive the symbol size:
As you mention, it's not an ideal workflow to manage with a lot of layers. I'm not familiar with the Pro SDK, but I imagine you should be able to write something to manage this, or manage it with some Python code. However, as a proof of concept this should at least let you figure out if this workaround will work for you.
Alternatively, you can take the symbol definition JSON that this creates when the MMPK is generated, and modify that in the runtime to update as necessary, using AGSSymbol.fromJSON() to create a new symbol to apply to the layer. That's the code I have to work out how to dig up.
Cheers,
Nick.