The beauty of the Arduino is the ecosystem behind it. The original hardware is verging on obsolescence, to be polite, but the same code and programming methods can be used on STM32 hardware (that's Arduino-compatible, like the Blackpills) or ESP32. The main difference is that now you have much faster, more modern processors, far more Flash and RAM to play with, not to mention a more powerful I/O peripheral structure if you want to dig into it at that level.
FreeRTOS on an ESP32 in the Arduino IDE is effectively free: you don't have to do anything to enable it. It's pulled in by default.
AFAIK, Arduino support for STM32 is limited to the F103 & F4xx series.
I confess that since my entry point is as a professional, I really haven't kept up with what other hobbyist-level entries are still on the market. That said, a good place to start would be with an STM32 Discovery board -- if you can find one these days! Looks like Digi-Key has exactly 1 in stock rn. It's a $19.95 board with an 'F407 and some sensors and output devices. No external tools needed: you can program it through a USB port. All the tools can be downloaded from ST Thomson or its partners for free. This is more entry-level professional than hobbyist, but there's no sharp dividing line there.
FreeRTOS on an ESP32 in the Arduino IDE is effectively free: you don't have to do anything to enable it. It's pulled in by default.
AFAIK, Arduino support for STM32 is limited to the F103 & F4xx series.
I confess that since my entry point is as a professional, I really haven't kept up with what other hobbyist-level entries are still on the market. That said, a good place to start would be with an STM32 Discovery board -- if you can find one these days! Looks like Digi-Key has exactly 1 in stock rn. It's a $19.95 board with an 'F407 and some sensors and output devices. No external tools needed: you can program it through a USB port. All the tools can be downloaded from ST Thomson or its partners for free. This is more entry-level professional than hobbyist, but there's no sharp dividing line there.
The Raspberry Pi pico is also taking off: https://www.raspberrypi.com/products/raspberry-pi-pico/ I haven't used one yet, but they seem to be amazing little devices and the community is rallying around them.