Even a Cicada Becomes a Dragon If Reborn蝉だって転生すれば竜になる

Burn the letters and curse them to lead them to eternal damnation and never reincarnate ever

Burn yourself, go the Chinese or Greek / similar underworld, find them before reincarnation or eternal damnation or anything else, and then give them the written letters.

If they unfortunately are not of the ethinic or religious line mentioned above, it will become more difficult due to the fact you will need to follow certain choices for an undetermined amounts of time to get to the places they have gone, using this method will, unfortunately, not allow you to get all of them, unless you spend multiple lifetimes on this question, even so it's unlikely.

Don't forget to contact me from down under or up or anywhere you end up using the internet, I guess.

the shortest one I had seen was "The Lazy Dragon Is Working Hard/怠惰なドラゴンは働き者" 15 chapters + epilogue