I personally prefer Mabie just because of all other “not really secret” trails that are there, though the red loop itself is still quite fun.
Haven’t been to Dalbeattie for a while but I remember quite liking some of the sections, quite a it of fireroad if I remember rightly.
How about Drumlanrigg, it’s a bit natural feeling for the most part and if you have the energy afterwards you could always do Mabie too.