Also came up with this based around Cairngorms but over distance.
635km, 10,169m
https://www.openrunner.com/r/10937459
Might add more distance but IIRC the Giola Dam road (kennock to pubil) is hillier than Ben Lawers.
https://ridewithgps.com/routes/31827093EDIT: Pish, it is hillier to get to Bridge of Balgie directly but not once you add in the leg to Pubil and back when going over Lawers
It's not adopted but it is passable on a bike, though the steep switchback is a bit loose.
Golf course road into Braemar is IIRC marginally shorter Edit: But you lose 5m of climb for 100m less distance
There's some really weird spikes and loops showing up though.
Some of the turnpoints are rather featureless, on a good day at the top of the Errochty climb you'll get a picture with some non-descript trees, on an average you'll get mist.
Although I do note that we're getting into the practicalities of proving you got to places rather than getting the appropriate climb and distance!
Instead of going to Fettercairn once over CoM, you could turn right at the bottom of the hill (Clatterin' Brig) and go up towards Drumtochty Glen a bit for some climbing then turn about there. This will trim distance and probably add some climb.
6km from the milk bar to Fettry arch, 15m climb, 76m lost
Turning up drumtochty gets you 63m in 2.5km