The iPad 12.9” would be excellent for long typing sessions. In fact, it has a True Tone enabled screen that is vastly superior to the 12” MacBook and 13” MacBook Air screens. As for the keyboard, there are several full size keyboard options, including Apple’s own Smart Keyboard, any Bluetooth keyboard, and even any USB keyboard.
Also, the 2017 and 2018 iPad Pros are excellent video editors, with the 2018 models having similar raw performance to the 15” MacBook Pros, and the 2017 models superior in many ways to the MacBooks and MacBook Airs. The thing is it appears Apple designed its SoC specifically to handle video editing workloads among other things, whereas on Intel Apple is using general purpose CPUs and is having to fight with its own legacy designs in Final Cut.
Put it this way: According to many video editors, while Final Cut on a Mac is more powerful in some ways and is entrenched for editing big and medium sized movies, video editing on an iPad Pro can be much more efficient, especially. for smaller shoots when time is key, so much so that some who have switched from a iMac/MacBook Pro to an iPad Pro have changed their advertised turnaround times from two days to one day.
At this point the two major missing features for a complete package in an iPad Pro is robust external storage support and mouse/trackpad support.