So are you saying that you have found in your studies that there is in fact not much of a historical precedent for the use of letter-writing for spread of philosophical ideas before Epicurus?
The use of literary letters in the Greek world is older than Epicurus though not very much older. This genre was originally developed by ancient orators and it was adopted by philosophers (Epicurus was also trained in oratory). Of course the distinction between orators and philosophers was muddy back then. That's why the professional orator and statesman Cicero is also labeled a 'philosopher' even though we was not much of an original thinker.
Paul was a missionary theologian but the distinction between theologians and philosophers also tended to be murky in antiquity. This is seen for example in how Christian theology adopted many philosophical terms like e. g. the Stoic term pronoia (providence) and changed their content. Just like Paul's letters (which were read during mass for the purposes of catechism) so did the letters of Epicurus function as materials for philosophical catechism.
The fact that the only specimens coming directly from the pen of Epicurus other than the Kyriai Doxai that survived intact are such letters indicates that their use was more popular among Epicureans to gain new converts and educate themselves compared to his many other authored texts which were voluminous and highly technical and as such not suitable for mass consumption. The Kyriai Doxai is also highly convenient so it's no wonder this managed to survive too.
Epicureanism had a mission and Epicureans were highly motivated to do what it takes to get as many people as they could to hear their message. Epicurus didn't think of his philosophy as a philosophy like any other. He thought of it as the only true philosophy and the only one of practical and real value.
It was this missionary-like spirit that goaded Diogenes to take up the expense of erecting a massive rock to transmit Epicurus' message to coming generations. I don't know any other example of people using inscriptions to spread an entire belief system and I don't know any other tradition before Epicureanism that did exactly the same thing using literary epistles. The use of epistles for catechism can't possibly have escaped the notice of non-Epicureans if Epicureanism was indeed popular back then (as it was). But nobody would have had any motive to copy this practice unless they also had their own message to spread to everybody and a strong missionary zeal to go with it. The early Christians were precisely such a movement.