Proof-of-concept for setting and managing a 30-minute session ID as a GTM Custom Javascript Variable.

Session ID - Cookie.js

function(){
  const cookieName = "session_id_with_expiry";
  const expirationTime = 1000*60*30; // 30min
  function genId() {
    return ([1e8]+-1e3).replace(/[018]/g, c =>
      (c ^ crypto.getRandomValues(new Uint8Array(1))[0] & 15 >> c / 4).toString(16)
    );
  }
  function getCookie(name) {
    var match = document.cookie.match(RegExp('(?:^|;\\s*)' + name + '=([^;]*)'));
    return match ? match[1] : null;
  }
  var cookieValue = getCookie(cookieName);
  if(cookieValue == null){
    cookieValue = genId();
  }

  var date = new Date();
  var dateTimeNow = date.getTime();
  date.setTime(dateTimeNow + expirationTime);
  var date = date.toUTCString();
  document.cookie = cookieName+"="+cookieValue+"; SameSite=None; Secure; expires="+date+"; path=/; domain=." + location.hostname.replace(/^www\./i, "");
  return getCookie(cookieName);
}