They will use zero-day exploits to get around the security barriers provided by "vanilla" smartphones. In GrapheneOS there are far less likelihood of zero-day exploits than any other phone, since AOSP and Pixel itself is subject to a lot of security researchers all over the world more so than any other phone. There are way more vulnerabilities in the OSes and all shitty bloatware of a standard smartphone from e.g Samsung or Huawei. AOSP and Pixels are battle-tested and on top of that, add GrapheneOS extra hardening of AOSP with the sole purpose of mitigating whole classes of vulnerabilities and 0-days.
I actually do not think the police authorities will have the means of just willingly access all of your data, camera/microphone and location on a Pixel with GrapheneOS. If you are that high profile of a target they have a jam-packed toolbox of other tried-tested means of spying on you that does not involve your smartphone.
If they have the power to do so, it will most likely not gain full-access to anything on the phone or is captured by the phone's sensors, but only partial access with some level of privilege escalation. Even if they had access to one of those zero-day exploits, remember those are worth a big load of money, and they will not risk of burning one of that exploits on the average joe in mass scale, in case a researcher finds out. They can only use them for very special targets. This is of course true if you have good security practices and use the features of GrapheneOS in a smart way. Of course if you install a bloated app full of security holes and give it all permssions, then Graphene can do nothing about it.
When the police say they can hack your phone and access your microphone and camera, it does not mean that they need to hack into the base layer Android system. It can very well be one of the other pre-installed or commonly used third-party apps that most people use. They probably have access to a plethora of unknown zero-day exploits in all kinds of Android/iOS software that many people, including criminals, use every day.
I would say easily, go for it! Get a Pixel ASAP and flash GrapheneOS on it immediatley. Do not forget, even if there is a possibility that the French police can access your data, there are far more adversaries out there than the them. GrapheneOS indeed does a very good job of protecting your private life at least most of the time. You will see how big of a change this is for your digital wellbeing. There is no other system like this out there.