Merge two tables: Right on Left and append all unique rows from Left

all from my guess

let
    Left = Excel.CurrentWorkbook(){[Name="Left"]}[Content],
    Right = Excel.CurrentWorkbook(){[Name="Right"]}[Content],
    Custom1 = let
                 col=Table.ColumnNames(Left),
                 LeftAntiGrp=Table.Group(Table.NestedJoin(Left,col,Right,col,"n",JoinKind.LeftAnti),"Column1",{"n",each _}),
                 RightGrp=Table.Group(Right,"Column1",{"n",each _})
              in
                 Table.FromRecords(
                                   List.TransformMany(
                                                      Table.ToRecords(LeftAntiGrp),
                                                      each let
                                                              a=RightGrp{[Column1=[Column1]]}?[n]?
                                                           in
                                                              if a=null then Table.ToRecords([n])
                                                              else List.Transform(
                                                                                  List.Zip(List.Transform(
                                                                                                          {[n],a},
                                                                                                          Table.ToRecords
                                                                                                         )
                                                                                          ),
                                                                                  each if _{1}=null then _{0} else _{0}&_{1}
                                                                                 ),
                                                      (x,y)=>y
                                                     ),
                                   col
                                  )
in
    Custom1

this is truly bananas... 🙂

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSiwoyElV0lFyTEnMBVKGRkqxOtFKSYl5QAgSLsrLz0kBMizA4jmpufl5QF5wYk5OJZC2BIsGpCYWATk+mXkpID2GyIKuRZnJIDEDZEFnoEqQdkMzpdhYAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t, Column3 = _t]),
    Left = Table.AddIndexColumn(Source, "Index", 0, 1, Int64.Type),
    Right = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSiwoyElV0lFyTEnMBVLm5uZKsTrRSkmJeUAIFPDNTM5ITM0BsoyMwTI5qbn5eUBucGJOTiWQtgSLBqQmFgE5XvkZIDlDZDEFMI6NBQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t, Column3 = _t]),
    #"Added Index" = Table.AddIndexColumn(Right, "Index", 0, 1, Int64.Type),
    #"Merged Queries" = Table.NestedJoin(#"Added Index", {"Column1", "Column2", "Column3"}, Left, {"Column1", "Column2", "Column3"}, "Appended Query", JoinKind.LeftOuter),
    #"Added Custom" = Table.AddColumn(#"Merged Queries", "Custom", each Table.RowCount([Appended Query])),
    #"Filtered Rows" = Table.SelectRows(#"Added Custom", each ([Custom] = 0)),
    #"Appended Query" = Table.Combine({#"Filtered Rows", Table.SelectRows(Left,(k)=> not List.Contains(#"Added Index"[Index],k[Index]))})
in
    #"Appended Query"