You can't really add a route to Waze, because it doesn't quite work like that. You can make sure all the roads are there, but Waze will then route you based on what it knows about the speeds that it has "seen" on those roads and according to it's algorithms. It's really designed to get you there rather than viwe the scenery.
Generally speaking, the roads in that area look pretty complete. Most likely, your problem here is that Waze is programmed to prefer major roads for longer journeys. A little way after Kincardine O'Neil, your preferred route leaves the A & B roads and heads off north along an un-named street type. For a route of this length, Waze is unlikely to use street-type segments unless there is no alternative. Even selecting "shortest route" in the preferences won't override the preference for higher road types.
It is possible that the route is "broken" - a missing section or an invalidly restricted turn, but I suspect that this isn't the problem here. Anyway, we have some fairly good editors in that part of the country.
Since it's not a short route, the best person to check it is you - given that you know it. I would have to use 2 screens or keep swapping between the Google & Waze maps to be sure I was on course. Start here in the editor and press shift-Z to show all turn restrictions. Then move along "your" route and see if you can see any red arrows or missing segments that might block the route. You can find more UK-specific information in the UK Wiki (link in my signature) and there are links in there to the main Wiki with information on how to use the editor.
If I can be of more help, please post back to this thread, or start a new topic in the UK Forum. I don't live in Scotland, but I visit there every year. From my point of view, the places I visit in the Highlands are remote enough that I don't have this problem - there's generally only 1 route available.