I have just been through the process of getting a certificat d’immatriculation for a 2020 car brought from the UK. We used Avignon Automobiles which were very helpful. They charged 50€. They lodged the forms with ANTS 12 August and today we received the CPI, so we can now get the French plates.
Just thought I would add that the reason we went through an agency was we can’t yet use France Connect (no tax number yet) and all the places that the ANTS website suggest in our area were most definitely not interested in doing it. We even went to our car makes dealership and they wouldn’t do it either.