Issue with low voltage for mosfet gate
What made you think the IRF3205 was suitable? It is not a logic level MOSFET. For use as a switch, ignore the gate threshold voltage. This is the gate-source voltage where the MOSFET just barely starts to conduct. You don't care about that. You care about the MOSFET conducting as much as possible so  instead of gate threshold voltage, look at the gate-source voltage used to achieve the desired RDson.

You need a 3.3V logic level MOSFET so the 3.3V on the RPI GPIO can actually turn it on and off.