Like the official @GrapheneOS account has already indicated - this has nothing to do with security. If it was, they would have already allowed the GrapheneOS signatures.
I only used #GrapheneOS for two months and I already can see the OS and its development team are far more reliable and trustworthy than everything else on the Android ecosystem.
Totally. I have been using GrapheneOS since before the Pixels (Nexus era) and it has been a bliss. Every single day.
Plus they've already shown they can be trusted - wiping the signing keys when those were under threat to make sure malicious actors couldn't hurt the userbase. Mind you: this was before the foundation existed and AFAIK now impossible to happen again.