We found the solution in 100.1.0. After awaiting the ArcGISMapImageLayer.LoadAsync call, we are able to set layer orders, visibilities, renderers, definition expressions, etc. by working with the Sublayers property. Note that Sublayers is a list ArcGISSublayer objects, so we did have to cast a given layer to an ArcGISMapImageSublayer to set properties like Renderer, Opacity, and DefinitionExpression.
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