Good choice on the wire. It is low resistance and importantly low inductance for a long run. Some people will also appreciate your choice if you waster as much money as they did. 4mm^2 does not look like it will fit the holes in the connectors on that amp (judging from the pictures). Spade is a good way to go. Vampire makes several grades of spades, so if you go that way, verify what type they spec.
Personally, I would not waste the money on cryo treatment. There are many other things you can spend that money on that will actually improve the sound.
Stick to gold plate. Rhodium is great for plating jewellery as it is hard which you want for mechanical protection and unlike silver does not tarnish. For electrical contacts, you want malleable so the materials can deform and increase the contact area.
Personally, I would not waste the money on cryo treatment. There are many other things you can spend that money on that will actually improve the sound.
Stick to gold plate. Rhodium is great for plating jewellery as it is hard which you want for mechanical protection and unlike silver does not tarnish. For electrical contacts, you want malleable so the materials can deform and increase the contact area.