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.
If your optimizing setup is -Os by default, that means that, probably, you created your project through CubeMX; if you created it directly in System Workbench you would be using -Og, that yield less optimized but simpler to debug code.