In ArcObjects, we could call ISpatialReferenceFactory2::get_GeoTransformationDefaults() and then IGeoTransformationOperationSet::Set() to add GeoTransformations. Following that, calling Project() either explicitly or implicitly (e.g. when fetching a Raster with a different Spatial Reference) would use these as appropriate depending on the GCS of the Spatial References. Calling ProjectEx() was not required, and anyway would not be used in the implicit cases.
(In our particular case, we currently only need for an OSGB <-> WGS84 transform (e.g. esriSRGeoTransformation_OSGB1936_To_WGS1984Petrol) to be applied when converting between 27700 and 4326).
Is there an equivalent available in ArcGIS Pro SDK?