what's your reccomendation ?
However I was wondering what would be totally open source toolchain and development setup? That might be too advanced for me now (and I'm happy with graphical peripheral selection in ST tools), but would like to try it in the future
There are plenty of open source projects for STM32
There is https://www.openstm32.org
Also there are plenty of Open Source RTOS projects that come with a HAL, so you can write code that is vendor independent.
Zephyr is probably the largest full-featured RTOS, it's backed by the Linux Foundation and several hardware vendors (NXP, Nordic, Intel, Ti and SiFive), but the community provides support for other vendor's chips too.
RIOT-OS is more popular in the academic sector, targeting sensor networks.
NuttX tries to bring a POSIX environment to your MCU and of course STM32 is supported there too.
there is of course ARM Mbed, but that feels more like a framework than an integrated OS.
Heck you can even run Linux on the larger STM32s (M4, M7) if you attach some external RAM - kernel support was provided by ST themselves.