I recommend toying with an arduino. The Arduino itself can be obtained for less than 3$ on Aliexpress (Nano with a microusb is a good start) They don't have an

I recommend toying with an arduino. The Arduino itself can be obtained for less than 3$ on Aliexpress (Nano with a microusb is a good start) They don't have any OS, rather, your program runs basically on the CPU directly. You can try to develop a OS from there, the CPU supports almost everything you need; interrupts and timers. The only real limitation is that you have 2KiB of memory, on the other hand, it can be extended if you are willing to solder on a SRAM (2$ for 64KByte) or you can see it as a challenge to get an OS with a few processes working with such little memory. The chip also has some advantages; since data and code live in seperate memory, you can't really crash or damage the chip by going out of bounds in an array. You can program something fairly robust in an arduino.