Are there any standalone or spin-off volumes related to The Walking Dead comic universe, separate from the main 32?

Yes, there are several standalone or spin-off volumes. Notable examples include 'The Walking Dead: The Alien' (a one-shot), 'Negan Lives!' (another one-shot bridging events), and 'Clementine' (a young adult graphic novel series set in the Walking Dead universe, starting with 'Clementine Book One'). These exist outside the main 32-volume numbering.