This is my initial solution. Just have the event handler trigger your code wrapped as a function. That way you can just call that same function when updating via javascript.

As @dfsq has pointed out this is just how it works. Manually triggering the event would probably take more code than this method:

// All code originally in eventListener now in function
function onActivityChange() {
  console.log("I work triggered by html and js")
}

// Call above function from eventlistener
document.getElementById('activitySelector').addEventListener('change', onActivityChange)

// Trigger same function after updating value
document.getElementById("activitySelector").value = interactiveType
onActivityChange()