Generally USB3.1 A to C (or C to C), 5 or 10 Gbps, and 3A (at least, mostly enough, yet without exceeding recommended amperage) current cables should work well, but if only one or two cables needed choosing the one with E-marker chip (USB-IF certified) is the most reliable way to go.
Basically repeating what @dcz already pointed out (warned) about that cheap “thin wires” are not always the best “solution” to charge any smartphone of yours. In short, I think that consideration of buying longer charging cable (perhaps with E-marker quality plugs) isn’t bad thing if keeping original Librem 5 charger, as recommended from Purism side anyway.