If you never heard about Free-and-Open-Source ZEPHYR RTOS [1] (Apache 2.0 licensed) then it will make your day as the best platform out there for your new embedded electronics design :-) It works on anything.. even on Open-Source-Hardware RISC-V CPU [2] :-)

[1] https://zephyrproject.org/
[2] https://riscv.org/