What diameter would those flag halyards be? Say 8mm?? In which case, at 1:144 scale you need a filament 0.05mm diameter and you are not likely to find thread this size anywhere! If you are stuck on strict scale (as near as possible) then perhaps try pulling thread filaments from nylon sewing thread or get hold of some fine-weave silk cloth (the sort of stuff we used to cover the wings etc of our model airplanes back in the 40s and 50s!) and pull out filaments from this. It’s not going to be easy but just consider this: a 0.1mm thread at 1:144 scale is the equivalent of 144mm full size!@
. Good luck, Robin