It wouldn't consider having to flash an intermediate ROM "incredibly restrictive", that is quite the exaggeration. the fact that you can install a ROM is the opposite of restrictive. Maybe it's a minor inconvenience. Set it up for custom ROM's, flash marshmallow, then flash Nougat. You want to talk restrictive, buy a phone from Verizon or AT&T.
You may find that it works perfectly fine to skip a version when installing a custom ROM, I just wouldn't tempt it, they are suggesting how it was tested.
Like I said though, ask the question over at XDA or the OnePlus forum.