If you want to run full Foss system then there is pinephone with Linux OS and physical switches to turn off camera and microphone and other stuff. But it is still in very very early stage and it may increase your privacy but it will definitely reduce your security.
I understand camera and microphone access to malicious actor could be a valuable. But if you think your phone's camera is compromised you have bigger problems.
That being said, graphene OS is considered by far the most secure android OS. It has features to turn off camera, mic, sensors. But more importantly it has a lot of additional security features making it harder for anyone to break in. And that using Graphene is or any other is doesn't directly make you secure.
Yes I can agree to assume baseband and all the other proprietary firmware is running a malicious code. But they can't use it to do mass surveillance, because if they do someone will detect it sooner or later. And also this is not the right tool for mass surveillance. 99% people will and have already downloaded apps that track almost everything they do.
And if your threat model requires you to be safe from malicious firmware, then maybe smart phones aren't for you.
I remember myth busters made a video about that, something like if gas tank was almost empty it atpeast got ignited otherwise nothing even when shot at gas tank.