Glentrool isn’t shocking – it is maybe a 5 – 7 hour ride in the wilderness on fireroads. Also, I had an absolute blast on the small blue route. It is a blue route which really has kids in mind, and is great if you were say taking a 10 year old down some bermy route the first time. But it is also great for big kids too,
After saying that, it is probably not what you are after hence you should not prioritise it above the others.
Drumlanrig is a bit slippery in the wet, but these are very short sections that you can take as per your skill levels : )
Dalbeattie also has some cracking off-piste secrets that locals ride!!