I live very local to a lot of these roads. The A84 Callander to Lochearnhead - busy, fast, especially so at spring / easter weekends. Section along Loch Lubnaig is twisty with some blind summits especially through the Pass of Leny. Road surface is good to rough.
A85 Lochearnhead to Crianlarich - Steep long climb up Glen Ogle with bends and a big drop on the N side. Fast. Some very good straights after Lix Toll, so a very fast driven road.
A82 south of Crianlarich, a long downhill section which has been straightened out over the years, so a fast road too.
All those roads above - loads of tour buses and logging trucks.
Stronachlachar to Aberfoyle - Recently voted in the local press as Scotland's most potholed road (I kid you not). I have ridden it three times since last Autumn, most recently 2 weeks ago, it is still a bumpy, pot holed, twisting tooth rattler, handle with care (The Trossachs Ton which was supposed to go along this road was last year re-routed OUT of the Trossachs down towards Balfron to completely avoid this road, I heard.)
Many of the roads heraboots have had hundreds of potholes 'filled' with that corn flakes and treacle stuff they spray out a machine, which makes for still rough riding and also lots of loose gravel.
Personally I feel pretty safe riding the main routes on my own, but I think a large bunch could be dodgy, but I am not so experienced with large bunches.
But apart from that it sounds fine. The trouble is that there are not a lot of roads which go to make up a big loop, plenty of dead end quiet glens but no use to a sportive organizer.