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Copper or silver-based should be lower resistance. These conductive paints tend not to be very conductive, the carbon stuff is essentially making a thin-film carbon composition resistor. Good for repairing rear window defroster heating elements, not so great as a 0-ohm trace in a keyboard. For short (<1cm) wires it's usually not too bad, but with the amount of damage I'm not sure you'll be able to repair the thing.
It looks like it might be from a Model M-style keyboard. Unicomp sells those.
Unfortunately the shipping fees would kill me since I'm in Europe, I really wish I could buy a new membrane but it seems like I need to do the repair myself.
Would copper tape suffice as a low resistance trace?
Worth trying. It's already broken, you can't really make it much worse. It'd probably work, and worst case you're back where you started & paying for expensive shipping.
I would suggest soldering on thin wires, even a bit of copper is going to be orders of magnitude more conductive.
Don't think I can solder on plastic, plus it would create new thickness and I don't think that would help some sandwiched membranes, but I'm going for double sided copper tape, that should help I hope.