Picture of the starter relay? If it is Chinese, it may be backwards and you may need to swap the cables over.
Also take a really good, close look at the wiring harness, and any connectors, between the ignition switch and the starter.
It takes a tremendous amount of current to crank the starter, and turn the engine over. Look around the connections,
for discoloration, blackening, browning, green mould, etc,,. The solution is to bypass the connector, by soldering
new wires from one side to the other. If you or a friend have experience in electronics, it'l be easier.