Welcome back, Ian
A google for "60 gauge copper mesh" had the expected mixed results (mostly the wrong metals), but about 20th down on the fleabay hit, past the brass, stainless and aluminium examples, was an actual supplier of copper mesh. Not cheap, but probably better than trying to weave your own from the insides of stripped flex. I seem to remember slot cars from waayy back using metal braids as pickups. No idea at all if such still exist.
Worn brushes will have the effect mentioned, but their side effect is pitting of the commutator which makes matters worse. Being metal to metal, they do need lubrication, the downside is that any sparking will oxidize the oil, and the residue will give problems.