switchup Yes, using Google Play Services (GPS) would help (not sure how much though) but I don't know if Signal will change to using FCM without reinstalling it.
Privacy will take a hit but since those apps are treated as regular Android apps and the user has to grant specific permissions, you have a lot more privacy and more control than a stock Pixel or other OEM (Huawei, Samsung, OnePlus, etc.) stock Android. Not sure if security would be weakened but it does open another attack surface, I presume. In regards to apps talking to each other, they can even without GPS (AFAIK) if the developers allow them to communicate with other apps. Apps only can if they both agree to. So, if one app does but the other doesn't, they can't.
Just a note: please take everything I say with a grain of salt as I am not the most knowledgeable on Android or GrapheneOS. If one of the developers says something contrary to me or someone who knows more, take what they say with more weight.
You can also visit the GrapheneOS FAQ here: https://grapheneos.org/faq
And your English is good!