How do I program a set of columns to remain leftmost using Power Query?

Sure thing 🙂

The function from the post can be applied, for example, by passing a table as the first argument and the ordered list of columns to be placed at the left as the second argument, i.e.

TableRelativeReorderColumns( SomeTable, {"col1","col2","col3"}

I have attached a small PBIX example.

1. I placed the function code is pasted into a query TableRelativeReorderColumns

let
  Function = 
    (data as table, columnsToOrderLeft as list, optional columnsToOrderRight as list) as table => 
    let
      CurrentOrder = Table.ColumnNames(data),
      ReorderLeft = columnsToOrderLeft,
      ReorderRight = columnsToOrderRight ?? {},
      OrderedColumnsRemoved = List.RemoveItems(CurrentOrder, ReorderLeft & ReorderRight),
      NewOrdering = ReorderLeft & OrderedColumnsRemoved & ReorderRight,
      Reordered = Table.ReorderColumns(data, NewOrdering)
    in
      Reordered,
  FunctionType = 
    type function 
      (
          data as table,
          columnsToOrderLeft as (type {text}), 
          optional columnsToOrderRight as (type {text})
      ) 
      as table
      meta [
        Documentation.Name = "TableRelativeReorderColumns", 
        Documentation.LongDescription = "Returns a table from the input <code>table</code>, with the columns in <code>columnsToOrderLeft</code> appearing leftmost in the order given and the columns in <code>columnsToOrderRight</code> appearing rightmost in the order given. Other columns will not be reordered."
      ],
  Ascribed = Value.ReplaceType(Function, FunctionType)
in
  Ascribed

2. Then created an example query MyTable with the above function applied at the last step, placing columns "A","B","C" in order on the left:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTICYmMgNgFiUyA2A2JzILZQio0FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [x = _t, y = _t, A = _t, B = _t, C = _t, z = _t, p = _t, q = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"x", Int64.Type}, {"y", Int64.Type}, {"A", Int64.Type}, {"B", Int64.Type}, {"C", Int64.Type}, {"z", Int64.Type}, {"p", Int64.Type}, {"q", Int64.Type}}),
    #"Reorder Columns" = TableRelativeReorderColumns( #"Changed Type", {"A","B","C"} )
in
    #"Reorder Columns"

Note that this ordering will apply in Power Query, but in the data model it will apply only on initial load. To "reset" the ordering to match Power Query, you can unload then reload the table (a messy process). There could be some other method involving external tools as well.