Ppx_yojson_conv plugin silently ignores custom serialization/deserialization functions

Learning
,
3

I think the approach of defining a custom type is the most idiomatic one.
It may be slightly more safe to have something like

module Sec : sig
  type t [@@deriving yojson]
  val of_string : string -> t
  val to_string : t -> string
end

just to prevent accidental type conversion, and maybe provide extra documentation around the intentions you have with the type