A script which combines audible-cli and update_chapter_titles.py to create a 1-1 m4b file from an aax file.

convert_audible.sh

#!/bin/bash

# This script has a few pre-requisites:
#
# 1. The audible-cli must be installed and available in the $PATH.
#    https://github.com/mkb79/audible-cli
#    e.g. export PATH=$PATH:/home/$(whoami)/.local/bin
#
# 2. The update_chapter_titles.py must also be present in the same 
#    directory as this file. This also relies on python3 being in the $PATH
#    https://github.com/mkb79/audible-cli/blob/master/utils/update_chapter_titles.py
#
# 3. The $activation_bytes variable must be set with your own Audible account activation
#    bytes. You can get this using the audible-cli when logged in.
#
# 4. asins.txt should be a single-line space-delimited text file containing the ASINS of any
#    audiobooks you wish to download and configure chapters. These MUST be audiobooks in
#    your own collection.
#
# DESCRIPTION:
#   This utilises both mkb79's audible-cli and mkb79's update_chapter_titles.py scripts
#   in order to convert an Audible .AAX file into a DRM-free .m4b file. By default, 
#   converting just the .AAX file will result in unnamed chapters, so this script
#   will also grab the raw Audible chapters file and will convert it into a ffmpeg-friendly
#   metadata format, which is injected in during the conversion step.
#
# USAGE:
#   > ./convert_audible.sh <ACTIVATION_BYTES> <PATH_TO_asins.txt>
#
# NOTE:
#   Some audiobooks advertise a different number of chapters compared to what the
#   raw chapters JSON file contains. This will be flagged by mkb79's script. It is
#   generally safe to hit 'y' when this occurs. Most of the time this is due to the
#   Audible intro/outros being included in the chapter metadata.
#
#   Example:
#     Missmatch between chapter numbers.
#     Do you want to continue? [y/N]: y
#
#
output_dir=../
activation_bytes=$1
asins_file=$2
asins=$(cat $asins_file)

for asin in $asins; do

    mkdir -p current_job
    pushd current_job/

        echo "Grabbing audiobook..."
        audible download -a $asin --aax

        audiobook=$(ls *.aax)
        file_base="${audiobook%.*}"
        file_proper_fullname="$file_base.m4b"

        echo "Grabbing chapter metadata for $file_base"
        audible download -a $asin --chapter
        proper_chapter_meta=$(ls *.json)

        echo "Convert from $audiobook to $file_proper_fullname using $proper_chapter_meta"

        echo "Extracting current metadata from audiobook..."
        ffmpeg -i $audiobook -f ffmetadata old_meta
        echo "Extraction done..."

        echo "Converting old metadata (you may need to manually enter Y)"
        python3 ../update_chapter_titles.py --separate-branding -f old_meta -a $proper_chapter_meta -o new_meta
        echo "Metadata done..."

        echo "Ripping and converting $audiobook to m4b..."
        ffmpeg -activation_bytes $activation_bytes \
            -i $audiobook \
            -i new_meta \
            -map_metadata 0 \
            -map_chapters 1 \
            -c copy \
            "$output_dir/$file_proper_fullname"

    popd
    rm -r current_job/

done

You can use this to obtain the activation bytes and even convert smaller sized audiobooks (upto 1.8GB): Audible Tools