Why are my LED weak?
Two causes.

Power supply too weak. At 5 meters 5050 rgb or 5050 3 diode white you are looking at 1.2 Amp per meter for all the channels. So 6 Amps minimum power supply. For single diode white like 3268 you are only looking at 2+ Amps.

More important, the IRF520 is not suitable for 3.3V logic level applications. It is barely on at that gate voltage. Its resistance will be too high,  resulting in the brightness issue you see. At 400 mA is okay, but not at 2 Amps per channel.

Look for a logic level mosfet, or a mosfet driver circuit. A simple npn transistor setup would do it.