Finding first, second, third occurence

This could be another option for you:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTI0MACSlgZKsTrRSkYgASMkAWOQgCmSgAmQaYHEN0U3wgwkYIgkYI5uhAVIwARJwBJsBoIPZqNoMQS71AxZBOxUuEtiAQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Index = _t, Value = _t, Threshold = _t]),
    
    ChangedType = Table.TransformColumnTypes(Source,{{"Index", Int64.Type}, {"Value", Int64.Type}, {"Threshold", Int64.Type}}),
    
    AddAboveThreshold = Table.AddColumn( ChangedType, "AboveThreshold", each [Value] > [Threshold], type logical),
    
    Grouping = Table.Group( AddAboveThreshold, {"AboveThreshold"}, {"AllRows", each _, type table}, GroupKind.Local),
    
    ValueMapping = #table( type table [Index3= Number.Type, AddedValue = Number.Type], { {1, 1000}, {2, 2000}, {3, 5000} } ),
    
    fnAddValue = ( tbl as table ) as table =>
        let
            AddIndxCol = Table.AddIndexColumn( tbl, "Index2", 1, 1 ),
            MergeTables = Table.NestedJoin( AddIndxCol, {"Index2"}, ValueMapping, {"Index3"}, "JoinedTable", JoinKind.LeftOuter ),
            ExpandCol = Table.ExpandTableColumn( MergeTables, "JoinedTable", {"AddedValue"} ),
            FillDown = Table.FillDown( ExpandCol, {"AddedValue"})
        in
            Table.SelectColumns(FillDown, {"Index", "Value", "Threshold", "AddedValue"} ),
    
    ModifyTableCell = Table.AddColumn( 
                                Grouping, 
                                "AddValue", 
                                each 
                                    if 
                                        [AboveThreshold] = true 
                                     then 
                                        fnAddValue( [AllRows] ) 
                                    else 
                                        Table.SelectColumns( [AllRows], {"Index", "Value", "Threshold"} ),
                                type table ),
                                
    ExpandedAddValue = Table.ExpandTableColumn(
                            ModifyTableCell, 
                            "AddValue", 
                            {"Index", "Value", "Threshold", "AddedValue"}
                      ),
    RemovedOtherColumns = Table.SelectColumns(
                                             ExpandedAddValue,
                                             {"Index", "Value", "Threshold", "AddedValue"}),
                                             
    ChangedType2 = Table.TransformColumnTypes(
                            RemovedOtherColumns,
                            {
                                {"Index", Int64.Type}, 
                                {"Value", type number}, 
                                {"Threshold", type number}, 
                                {"AddedValue", Int64.Type}
                            }
                        )
in
    ChangedType2