Reference for https://experienceleaguecommunities.adobe.com/t5/adobe-analytics-questions/javascript-when-a-tracking-call-has-completed/qaq-p/380272

aelc_20200930.js

// default the flag to false (it gets set to true in registerPostTrackCallback)
var flag = false;

// wait for flag=true (set in registerPostTrackCallback) to fire the s.tl() call
var waitForFlag = setInterval(function() {
  if(flag) {
    console.log("fire s.tl() here");
    clearInterval(waitForFlag);
  }
}, 50);

// this is a placeholder for the s.registerPostTrackCallback)() logic;
// consider checking for the call type (make sure it's not an s.tl() call) before 
// setting flag=true
function pretendThisIsRegisterPostTrackCallback() {
  console.log("in registerPostTrackCallback; setting flag=true");
  flag = true;
}

// this is just a delay to fire the s.t() call after a couple of seconds;
// in your scenario, this would be the actual s.t() call
setTimeout(function() {
  console.log("fire s.t() here");
  pretendThisIsRegisterPostTrackCallback();
}, 2000);