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.
Hmm, apparently changing firmware in the standalone adapter ST-Link V2 also helped. My board is able to been reset and work standalone without the adapter connected.