I run a layout that is a bit bigger than that on a 4 amp supply but... I have another 4 amps running the accessories (point decoders etc.) so another vote for the accessory bus.
I don't know what decoders you use and whether they have an ABC braking facility but with a layout like this, I would set up ABC braking zones in all the terminus platform tracks. This reduces your chances of crashing into buffers when trying to control four trains on an end to end layout. I have done this with my branch line and now can leave my auto coach /Pannier or bubblecar trundling to and fro all day in shuttle mode. But like I said, you have to have decoders capable of doing this. Needs thinking about before laying track.