Are you doing production directly in MP3?
Stop that. MP3 causes permanent distortion, it gets worse at each edit, and you can’t stop it. Work in WAV files (in addition to projects if you wish) for Edit Master and Archive and make the ACX MP3 just before you submit.
Those aren’t the ACX specifications. It’s No Louder than -3dB and between -18dB and -23dB.
Instead of doing the sound processing manually, use the Audacity ACX Audiobook Mastering Macro. The Macro automatically suppresses rumble distortion, sets RMS (loudness) at -20dB (in the middle) and puts the peak value at -3.5dB.
Even better, you can apply it multiple times and you can’t hear it running.
Koz