You can use a second Pico as your debugger.
It's getting harder to source these with stm32f103's as time goes on, so you might be better off choosing a stm32f103 dev board
You can use DAPLink firmware with any of it's supported chips (LPC4322, LPC11U35, K20, K22, KL26). You'll need to use the 'develop' branch to use GCC to build it. You'll need to find a chip with the correct
Firmware source: https://github.com/ARMmbed/DAPLink/tree/develop