The implementation of spatial definition queries in ArcGIS Pro v3.5+ is a start, but seems limited, as others have noted here in ArcGIS Ideas, to intersect, etc. It's also apparent that you have similar functionality that is being used in Select By Location on the GUI.
A potentially valuable approach/improvement might be to implement expression files that can be saved from the Select by Location GUI dialog (saving to .exp file). This is already an option in the Select by Attribute GUI dialog, and those .exp files can be the basis for a definition query. So, if you make Select by Location .exp files available to the user and allow the user to string together multiple .exp definition files here with SQL, it could be a rather powerful approach to combine attribute and spatial queries together as a definition query (and in a way that is consistent with other elements of the Pro GUI).
