Fortigate 70G 7.4.X & 7.6.X Firmware

Yeah, most people do not read the Fortinet definition of "Mature" or "Feature". All "Mature" means is that they have stopped adding new features to it. Fortinet will NEVER use the word stable in reference to a version.

FG-70G is quite new, that why FOS 7.4 and 7.6 is not ready for it.

On the other hand, 7.6 is not mature yet and I'd not use it in critical production until it is mature.

For now you should update to 7.2.11 which mature, vulnerability free (so far) and very stable.

New hardware models, like 70G, when released have their own branch of FortiOS, separate from the main train ForitOS, as new models introduce new hardware/ASICs/etc and Fortinet take their time to test them out. Eventually, those new models cease to be "new", their separate software FortiOS branch is merged into main release branch, and after that they get FortiOS version releases just as any other FGT model.  Until then though, FortiOS release for such models come out at a much delay, compared to the main branch.  

As per post here https://community.fortinet.com/t5/Support-Forum/FortiGate-70G-firmware/td-p/377408 , Fortinet are planning to merge into main branch 70G anytime soon, starting with 7.4.8. If it indeed happens, you would be able to upgrade to 7.4.8, but until then - just 7.2.11, which is totally fine as others already said.