The answer here is likely generally not. Exploits that would achieve anything even remotely similar to what you're describing cost millions, and they're used in very specific circumstances.
GrapheneOS does implement measures to prevent exploitation of unknown vulnerabilities in multiple ways, at least some of which are documented here:
That, alongside the fact that GrapheneOS ships patches/updates extremely quickly, I would say you're more than likely fine.
If that isn't enough assurance for you, it's possible that your threat model is so extreme (and potentially unrealistic) that no digital device will be secure for you.