The way I did the Cairngorms one was I started by labelling the highest road passes / cols. Then I tried to join them together.
I have a zip of GPX with all the cols in GB but some are off road so you'd need to look on a map to find the ones that might be options.
Phil, IroIroMono and I are going to have a good sniff around this one and see if we can make it work properly, geographically speaking between us it's in our back gardens.
The biggest downside of it for me at least is the route passes my parents caravan far too early to be a sleep stop of a 3x200 route exploration.
There is a few tweaks that will need to be made of course.
Ideally I'd get the start down by the loch there's a couple of small developments that lead down to the loch right next door but both have rather loose roads.
I'm also aware of a route through Errochty forest to Ailean forest, but I've only ever been through when it's been snowy and icy so not sure of the surface.