Whatnoww As a follow-up to this, I tried using PGPP. Two things:
It doesn't allow you to use a VPN as I mentioned. This is actually stated on their website. In emailing directly with one of the creators, he is of the belief that the relay system they use is actually better than using a VPN because the information is not aggregated in a single place.
PGPP may provide an additional layer of privacy for a typical phone user, but it has some compatibility issues with GrapheneOS. Namely, use of multiple profiles (which is one of the best benefits of GrapheneOS). Because PGPP's "Relay" occupies the VPN tunnel on Android, it needs to be in each profile separately to work. But the app itself is not tied to any particular account, so from what I can tell you would need to install and pay for a separate subscription for each GrapheneOS profile.
In looking at PGPP's privacy policy, it's not great. It basically reads like a privacy policy from every major tech company there is. That is, it states they have a strong commitment to privacy and only collect the information they deem "necessary", but then they use broad statements to justify what "necessary" means. From what I can tell, they collect just as much information as any other carrier or tech company, and you'd have to take them at their word for what they do with it.
Best bet is probably to just try the Librem aweSIM and then use a VPN instead. It likely gives you the same level of privacy at a fundamental level but with a more transparent and reputable name.