The way I read the documentation, I was under the impression that I could set the Map's Spatial reference (when in Local mode), then point it at your Standard Basemaps and have it re-project those basemaps into my chosen Spatial Reference's shape.
From my (updated) understanding, we'd have to provide a separate Map Server endpoint for each of the Spatial References we want to support.
Are we allowed / able to re-project sections your Standard Basemaps using ArcGisPro into the various projections and expose as separate end points?