Phones are insecure, Pixel/GrapheneOS seems to be the best choice, but never trust phones fully.
The 3/4/5G chip is connected to other vital components outside GraphenesOS control and therefore Pixel phones should not be considered safe.
However, it's your threat level together with legal consequences that indicate what you can do with your pixel phone.
As the Graphene team states in their usage guide, "you can reduce attack surface by disabling 2G, 3 and 5G connectivity". The important words here are reduce attack surface, as like /u/DaVenom states, it is still a vulnerability, and one used quite commonly by LE.
Not the best idea to associate your online presence with any specific device in general. I would be hesitant to perform that specific action myself. As other users said, take your threat level into consideration.
While GrapheneOS does have wonderful features, it is not memory agnostic. This may not matter for the average user, but if you decide to use a pixel running Graphene to view hidden services via tor, just be aware that you are taking a risk.
The 3/4/5G chip is connected to other vital components outside GraphenesOS control and therefore Pixel phones should not be considered safe.
However, it's your threat level together with legal consequences that indicate what you can do with your pixel phone.