I’ve been working on a custom RTOS for Cortex-M for the past 10 years: https://github.com/raphui/rnk It started as a way to learn RTOS internals, and over time it has grown into something with lots of nice features. I’m even using it in a dirtbike anti-theft tracker I am building. Also, 2 months ago, I did a weekend challenge to build an embedded software parameter DSL and compiler. Its goal is to let firmware developers define configuration values, thresholds, constants, and other application-level parameters in a structured, human-readable format, and compile them into binary data that the firmware can directly use. https://github.com/raphui/epc

Happy to get any feedback :)