Can I use more diodes for lower voltage drop and higher current?
But the one I bought does not give me 5V, but 5.3V - that could cause trouble as I am going to power a Raspberry Pi 


There is nothing to worry about.
The USB spec allow up to 5.25 Volt on the host ports. Usual 5 Volt TTL logic allows VCC up to 5.5 Volt in most cases.

The Raspberry Pi has on-board voltage regulators, and uses the bare 5 Volt rail very little. 

You don't need a diode in your case, but the voltage drop of a Schottky diode probably would not hurt much either.