It absolutely is more private and secure than using stock OS. Don't trick yourself into making things complex and quitting. The mere fact that you're using GrapheneOS is a giant leap towards improving the security and privacy of your device.
Think of all the under the hood hardening, the privacy features like the network/sensors permission, contact/storage scopes etc.
All apps are sandboxed regardless of profiles. Apps can see what other apps are installed in a profile, and they can communicate with mutual consent. We're working on a feature that addresses this for the same profile, but using one profile is absolutely fine. I'd guess most people do.