Dynamically generate column schema as record

Now I probably see what you want to achive. You want to replace this part of Table.Group. Is that correct?

If yes, you can just delete that part of Table.Group and use Table.Combine to preserve column types, like this:

let
    src=Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("lZBBCsMwDAT/4nMiVivLtvSO3EL+/4267aUlhWJdBIJhR3ueZUSUrRD0uXaFjjCprfX3te6IHfVAJDxJ0WaYU67tg60v1jkqhCugzYXlJOuNTuntm1RPjGQVRL+Tr//oGOai3WyF/aGpI9GTsytiRdOPmaNMhlhd1PzH3jT9UOQslE08FjVnzpN0ib6seWevBw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [id = _t, year = _t, amount = _t, ing_est = _t]),
    ct = Table.TransformColumnTypes(src,{{"id", Int64.Type}, {"year", Int64.Type}, {"amount", type number}, {"ing_est", type datetime}}, "en-US"),
    GroupedRows = Table.Group(ct, {"id"}, {{"ad", each _, type table}}),
    Combined = Table.Combine(GroupedRows[ad])
in
    Combined