acl wrote:The question now is is it worth adding another ground plane on the reverse side of the board...
In this comparatively low-speed application a single ground-plane is enough. If this were running in the GHz region and using stripline techniques, then yes, but that's not the case.
It appears that generally the top (component) side is given over to the ground-plane, and the bottom (solder) side to as much of the actual trackwork as possible. There will be need to run some tracks on the component side, but that's only if it can't be done on the solder side. If your auto-router has priority settings set them up accordingly.
With a ground-plane I strongly advise using the solder resist option, otherwise accidental shorts while testing are almost inevitable. Silk-screening, nice to have, and doesn't add much to the cost (if anything). Likewise plated-through holes and vias. For components/pins connected to the ground-plane add 'thermal breaks' around the leads/pins to aid in reliable soldering...I'll try and find an example...
https://en.wikipedia.org/wiki/Thermal_relief
https://en.wikipedia.org/wiki/Copper_pour
Here pin 8 has a variation of the thermal break...more conventional, the larger pad, upper left...
Photo acknowledgement Hans Summers, QRP Labs.
Steve A.