Parsing list values in a nested table

Thanks ralf_anton and ronrsnfld for your suggestions.

I eventually managed to get what I needed. It may be that I didn't explain well enough what I was trying to achieve, but the revised code below creates a 'Reference_Date' column within the Transform File(s) that can be switched and manipulated using Process_Date and SysTimestamp.

Here is the code, and I hope it helps someone else trying to achieve something similar. Thanks.

let
Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WCg5yNjBU0lEyMjAyMzAxMoEydQ1MdI1MQgyNrAwMgChKKVYHQ60xCWqNcKqNBQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [FILE_CODE = _t, PROCESS_DATE = _t, SYSTIMESTAMP = _t]),
Change_Type_01 = Table.TransformColumnTypes(Source,{{"FILE_CODE", type text}, {"PROCESS_DATE", type text}, {"SYSTIMESTAMP", type text}}),
Add_Transform_File = Table.AddColumn(Change_Type_01, "Transform File",
each Table.TransformColumnTypes(
Table.FromRecords({
[Process_Date = "20260424", SysTimestamp = "2026-04-24T12:00:00Z"],
[Process_Date = "20260423", SysTimestamp = "2026-04-24T12:00:00Z"],
[Process_Date = "20260422", SysTimestamp = "2026-04-24T12:00:00Z"]
}),{"Process_Date", type text, "SysTimestamp", type text}
)
),
Add_Reference_Date = Table.TransformColumns(Add_Transform_File,
{"Transform File",each Table.FromColumns(Table.ToColumns(_)&{List.Transform([SysTimestamp],each Text.Middle(_,0,4)&Text.Middle(_,5,2)&Text.Middle(_,8,2))},Table.ColumnNames(_)&{"Reference_Date"})}
)
in
Add_Reference_Date