so formatting the 3DS seems to be the proper way to not bricking it when downgrading. but how to get my content (the most important are the saves) back to the newly formatted and downgraded 3DS? simple copy and paste won't work as there is a new AES key used to encrypt the new formatted 3DS. so I have to save them using homebrew before the format, right?
Yes you would use sdvt: https://gbatemp.net/threads/svdt-save-data-explorer-manager.396562/
to backup and reinsert the saves.