5ft between the rafters: 3ft for you 2ft for the layout, in the width.
Drawn as 11ft long. An extra foot would get an extra coach length for your trains.
You had drawn four points and mentioned perhaps two more.
The magenta track keeps to that six point limit for stage one.
You can have four trains taking turns on the single mainline, given the two long end sidings (dual track to be) and the "hidden" passing loops.
The next stage would be either to develop the station or to complete the double track as shown in light blue.
In each case either installing future points or matching short pieces of track as you go would minimize later disturbance.
Just thinking, Chris