Scott Why there are no "[ROM][UNOFFICIAL]" builds of GrapheneOS? [...] I prefer more Desktop like hardware (example Pinebook), but there is no GrapheneOS which seems the most secure OS.
A lot of the security of the OS depends on a foundation of strong hardware security. Some devices, including some Samsung and Apple devices, have more or less reasonable hardware security, but the manufacturers do not publish enough information for third-party operating systems to make use of that security.
Further information: https://grapheneos.org/faq#future-devices
See also the "Many other devices [...]" text here: https://grapheneos.org/faq#supported-devices