Is the xt90 the load? Gauges look good, even a bit overkill. Get high strand count tinned silicone wire. Check acerracing super worm. Prices are okay, US store, good reputation. Their ebay store may be a bit cheaper.
Don’t use copper rod – I think vibrations will break the joints.
Your pack is specified for 20a per cell (60a, 3p). For nickel I think .20mm x 10mm per series connection will be close to meeting the spec.
you should make your motor wires bigger than your battery wires if you want to have the wire sizes in relation to the amps running within them. Motor wires will have up to maybe 4x more amps depending on the esc settings. the amps run circularly through the motor and esc not to the other side with the battery and capacitors.
the current is regular standard current. the real thing.
a lot of people are ok with little motor wires and it’s the norm but none the less that’s where the majority of the amps are. at least there’s 3 phase wires so any of the single phase wires will only experience this higher current for half the time.