Unpivot or unstack columns dynamically

Try this:

It works on your sample

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bY0rDoAwEESv0qxGMOUvoT0DpqmowGER3J6diq1BvGYy3cxLSWJ5HaSTs9zP5XZLh6UguUvCH955ZagNL0ZlUubaBE2Lsipbbbjuf9aDpdjWez50wJsANIAKNAcoAS1QTf4A", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t, Column3 = _t, Column4 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{
        {"Column1", type text}, {"Column2", type text}, {"Column3", type text}, {"Column4", type text}}),

//Add column to group by dates
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Day", each if Text.StartsWith([Column1],"Day") then [Column1] else null, type nullable text),
    #"Filled Down" = Table.FillDown(#"Added Custom",{"Day"}),

//Unpivot each subgroup
    #"Grouped Rows" = Table.Group(#"Filled Down", {"Day"}, {
        {"UNPivot", (t)=>
            let 
                #"Promote Headers" = Table.PromoteHeaders(t),
                #"Removed Columns" = Table.RemoveColumns(#"Promote Headers", List.Last(Table.ColumnNames(#"Promote Headers"))),
                #"Renamed Columns" = Table.RenameColumns(#"Removed Columns",{{List.First(Table.ColumnNames(#"Removed Columns")),"Category"}}),
                #"Unpivot" = Table.UnpivotOtherColumns(#"Renamed Columns",{"Category"},"Type","Value"),
                #"Value to Number" = Table.TransformColumns(#"Unpivot",{"Value", each Number.From(_)})
            in 
                #"Value to Number",
            type table[Category=text, Type=text, Value=number]}
        }),
    
    #"Expanded UNPivot" = Table.ExpandTableColumn(#"Grouped Rows", "UNPivot", {"Category", "Type", "Value"})
 
in
    #"Expanded UNPivot"