At the end of the road for this project, I've moved on and started the Funktion Generator project, this project that will utilise multiple microcontrollers with one doing DDS synthesis and anothiner driving the UI, this allows for adjustments on the fly etc. Good progress has already been made, with the power supply (with +-9V rails) built and working, and the firmware for the DDS generator running and accepting commands vi I2C/TWI.

See: https://hackaday.io/project/19866-funktion-generator