Just an idea for folk but the forest road sections from Glen Trool to Clatteringshaws may be heavily cut up at the moment due to forestry work/wood lorries used to clear the area of Larch die back disease so a 28mm tyre at the least would be needed, if i’m up that way on the Tripster i tend to use my 40mm Clement MSO as it’s perfectly fast enough on the roads and gives a bit of comfort on the forest roads – the other roads are the usual mixture of rough rural surfaces (some bad, some very rough, some butter smooth) so a 28mm treaded tyre at the minimum would be preferable.
For practically all my riding throughout this area whether that be road or off road tracks/forest roads/paths i fit my Clement X-Plor USH , I’m not fussed about how fast or beating previous times though so my advice may be tarn with a pinch of salt.
(apologies for always bleating bout clement tyres but i’ve found them to be perfect for my type of riding in this area)