You're wrong. Wilbrand (the more popular alternative to LetterBomb) works on any System Menu. There are also easier methods such as BannerBomb. Have you done ANY research at all?
ModMii is the easiest way. You can stay on 3.2 if you really want to, but you'll miss out on running channels from the SD card through the System Menu and the Bannerbomb exploit.
Believe me, I've done plenty of research. I just haven't seen a single tutorial on how to get the homebrew channel that doesn't involve LetterBomb. Not my fault.