Reading to a new table only the text values

You can do a single column like this:

Table.TransformColumns(
    Source,
    {{"Col1",
      each if (try Number.FromText(_) otherwise null) = null then _ else null,
      type text
    }})

To transform all of the columns, we can make it more dynamic as follows:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8i9KN1TSUTIFYmcXIKEUqwMWNAKyDQ1AAjpKBjBBYyDH0QlIGBlA1MfGAgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Entity = _t, Col1 = _t, Col2 = _t, Col3 = _t]),
    fn_num2null = (txt) => if Value.Is(try Number.FromText(txt) otherwise null, Number.Type) or Text.Length(txt) = 0 then null else txt,
    TransformList = List.Transform(Table.ColumnNames(Source), each {_, fn_num2null, type text}),
    #"Transformed Columns" = Table.TransformColumns(Source, TransformList)
in
    #"Transformed Columns"