Renaming multiple Headers

Okay, try this as a custom step instead:

Table.RenameColumns(
    previousStepName,
    List.Transform(
        Table.ColumnNames(previousStepName),
        each {_, if Text.StartsWith(_, "EF") then Text.BeforeDelimiter(_, ")") else _}
    )
)

Working example query:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSlTSUUoC4mQgTgHiVKXYWAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"EF-1.1.1) Some text" = _t, #"EF-1.1.2) Some Words" = _t, #"EF-2.1.1) Some Writing" = _t, #"AB-1.1.1) Some Stuff" = _t, #"TT-1.1.1) Blah Blah" = _t]),
    
    Custom2 =
    Table.RenameColumns(
        Source,
        List.Transform(
            Table.ColumnNames(Source),
            each {_, if Text.StartsWith(_, "EF") then Text.BeforeDelimiter(_, ")") else _}
        )
    )
    
in
    Custom2

Example output:

Pete