Flying messenger gronti work on multiple scales because one messenger gronti carrying messages improves communications, and as more are built communication incrementally improves until you get to the scale where you can have networks of them flying regular routes with relays between them to form mail delivery systems.
Look, you said yourself:
All proposed solutions run into Rule of Prode issues
Networks aren't an option. And of all the discussed solutions, they're probably the one with the lowest capacity to transfer a high volume of mail due to being comparable speed to train like solutions but with far lower capacity and a Master Runesmith requirement on the ability to create them unlike semaphore designs. Even stuff without a rune involved such as traditional wagons or canal boats, would provide better volume capacity because even if they were a hundred times slower than a gronti, they can definetly carry more than a 100 times more.
These aren't owned by the post office. They're the personal property of the king who commissioned them. They fly 100's of miles there with a message and then fly back probably with a response.
They aren't doing routes, they aren't making a delivery system.
You seem to have completely misunderstood the Rule of Pride.
Snorri interprets it as not applying for lesser runes, because its just not practical for him to abide by it and still contribute to the extent he wants.
Vragni, knows dozens of iterations of runes that are just slightly different enough that he can justify calling them different runes and thus not subject to the Rule of Pride.
Apprentices are exempt, because they're still learning, so making mistakes and needing to try again is an inevitability and so they shouldn't be taking the same sort of pride in their work that they should after they've graduated.
Its not just something special about the Rune of Light.
The work of an apprentice like Snerra was enough that members of the clan swore a debt that needed to be repaid. When we were making Stormwrath for the KoS it was mentioned we used so much Gromril it could bankrupt entire clans. Snorri built a small city and the only real cost to him was that his money pile was noticeably smaller. I really really need to stress this, the only things that approach the cost of things Runelords have made will be stuff thats a couple of degrees of separation.
Non dwarf shaped Gronti's require a unique level of specialisation and skill that most Runelords do not have. Flying is likely to restrict this even further.
They're likely to either require Featherweight, which is only known by people Dolgi have given it to (and this includes an Oath not to spread the knowledge) or people who have Adamant for its superior strength weight ratio. Restricting it even further.
Its important to understand all this because although we mostly talk about the Rule of Pride in terms of limiting development, if you want to compare infrastructure costs, bespoke flying Gronti's each requiring the highest quality of runelord and material to make is actually likely to exceed the cost of a semaphore for equivalent mail carrying capacity.
