For just the rewards, you can dump your romfs files and use Paragon, and manually edit each reward to require 0 ranking, but you need Windows or MacOS. Don't get the fe10 dialogue add on version, it probably won't work. This is the easiest method.
Or, you use FEFTwiddler and manually put each reward into your convoy using your save file. Works on Windows, don't know if it works on other OS.
You might be able to change your renown by hex editing your save file, but I have no idea how to do that, or if it's even a method.