Development tools for Zephyr on STM32Workbench for Zephyr: VS Code extension to set up, build, flash and debug Zephyr applications on STM32.Devicetree manager: browse, edit and visualize Zephyr devicetree, STM32CubeMX-like for Zephyr.
Yup you were right. Thought I might have been using the wrong board configuration. Ok so I changed the stm32f4discovery.cfg back to how it was, and created a new project with stm32f407-disc1 and it still works.