After successfully developing the conlang, perhaps you can teach it to others, or others can try to attack it with a forensics or cracking roll?
Going to give 3 advantages to having a coded system, and 3 disadvantages to not having a coded system.
1/1. In an uncoded system, if I come up with a cipher, a GM would have to learn this cipher or conlang, or be provided a tool to translate said cipher OOCly. In a coded system, this is mitigated by allowing the GM to test on the NPC's or on your character the strength of the system, There is no OOC mechanic needed.
2/2. There is no OOC mechanic needed, meaning that people cannot play above or below their stats, which for a soft stat like intelligence, means many things. There being no OOC mechanic also opens up this to people who aren't interested in constructing their own tradecraft conlang from scratch.
3/3. In a coded system, there is no piericability of your ciphers without said ciphers being countered by a skillroll, putting this skill roll on forensics or cracking, puts it within two very underutilized skills and provides them something to use.
4/4. This removes the relativistic desire to pickup languages on a character who might otherwise ICly not be the type to be a polymath or polyglot. And would allow for greater flexibility and differences in languages amongst cliques and power groups but not the kind of security that cannot be breached by sufficiently motivated hackers/investigators.