I generally recommend going for the highest-spec board that suits your requirements. The STM32L4+, STM32L5, and STM32H7 are all amazing microcontrollers.
I'm playing with STM32 Cube IDE right now. I got surprised that it works well on Linux (ok, I did only basic stuff, but did not have any trouble setting it up).
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
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.
Are any of those available multi-core? It's one thing that's pretty nice on the ESP32, especially for the cost and lack of complexity for simple cases. I couldn't find a similar STM32 part that was relatively cheap, low pin count (eg 48 pin with ethernet Mac support), and had dual core.