Many of the new F-series devices announced last year do not yet have mainline FortiOS builds. I share in your frustration.
Consider that FortiOS 6.4 and the FG 1800F were both announced last spring, yet the 1800F could only run select 6.0 and 6.2 builds until this past July when 6.4.6 came to the 1800F - nearly a year and a half late. From what I understand, there were some issues getting 6.4 and up running on F-series devices with TPM, but this may just be a rumor (I don't believe the 1800F has a TPM, but the 200F does).
With that said, always be mindful in the future before you look to upgrade hardware that the version of FortiOS you're running on your current hardware is supported on the hardware you plan to upgrade to. We've done a few config backports for E-series customers, while others bought their new F-series devices and shelved. Not always an ideal scenario.
Hopefully they'll get a unified release of 7 soon. A few of our clients have it in production and it's been surprisingly stable for them considering it's so early on. We chalk this up to the focus on refinements of a select feature set in 7, as opposed to adding a ton of new features all over the place. I remain an optimist.