Give Céu a look[0]. It was specifically designed for embedded computing, and compiles to C, making it very easy to interface with existing libraries. It requires no manual memory management, and has very nice structured synchronous reactive concurrency primitives.

(EDIT: I realise that multitasking and concurrency aren't quite the same thing; it also has experimental interrupt support though)

Here's a made-up example for Arduino (for which it has support out of the box[1]). It creates two concurrent fading LEDs at different frequencies, and resetting at the push of a button:

    #include "arduino/arduino.ceu"
    
    input  int PIN_02; // button input
    output int PWM_05; // LED outputs, remember that pins 5 and 6
    output int PWM_06; // have a higher PWM frequency on the UNO
    
    // a code block that concurrently fades an LED in and out
    // `pin`   the output pin of the LED
    // `min`   min value of the fade
    // `max`   max value of the fade
    // `delay`   number of milliseconds to wait between increasing/decreasing `analogWrite`
    code/await Fade_forever(var u8 pin, var u8 min, var u8 max, var uint delay) -> void do
      loop do
        var int i;
        loop i in [min->max] do // fade in loop
          if pin == 5 then
            emit PWM_05(i);
          else/if pin == 6 then
            emit PWM_06(i);
          end
          await delay ms;
        end
        loop i in [min<-max] do // fade out loop
          if pin == 5 then
            emit PWM_05(i);
          else/if pin == 6 then
            emit PWM_06(i);
          end
          await delay ms;
        end
      end
    end
    
    loop do //endless loop
      // if *any* of these three code blocks (trails) end, all of the remaining trails in
      // a `par/or` are aborted and code resumes (in this case, the loop restarts).
      // By comparison, a `par/and` construct would require *all* of the trails to
      // terminate before continuing.
      par/or do
        await PIN_02; // .. meaning that if a button is pressed, we reset the loop
      with
        // fade the LED at pin 5 quickly between 64 and 192
        await Fade_forever(5, 64, 192, 5)
      with
        // fade the LED at pin 6 slowly between 0 and 255
        // note that because it fades over almost twice range (255 vs 128)
        // but not exact, it's around eight times slower, not four, and it
        // will slowly go out of sync. 
        // We can push the button to reset however!
        await Fade_forever(5, 0, 255, 20)
    end

[1] https://github.com/fsantanna/ceu-arduino