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

Session ID - Local Storage.js

function(){
  const storageName = "session_id_with_expiry"; 

  function genId() {
    return ([1e8]+-1e3).replace(/[018]/g, c =>
      (c ^ crypto.getRandomValues(new Uint8Array(1))[0] & 15 >> c / 4).toString(16)
    );
  }

  function setSessionId(name, value, ts) {
    var item = {
      "value": value,
      "expiry": ts
    }
    localStorage.setItem(name, JSON.stringify(item));
  }

  function getSessionId(name) {
    const ttl = 1000*60*30; // 30 mins
    const date = new Date();
    const dateTimeNow = date.getTime();
    const ts = date.setTime(dateTimeNow + ttl);
    var id = localStorage.getItem(name);
    var item = JSON.parse(id);
    if(!id){
      setSessionId(name,genId(),ts)
    } else {
      if(dateTimeNow > item.expiry){
        setSessionId(name,genId(), ts);
      } else {
        setSessionId(name,item.value, ts);
      }
    }
    id = localStorage.getItem(name);
    item = JSON.parse(id);
    return item.value;
  }

  return getSessionId(storageName);
}

This variable will update the 30-minute expiry with every action on the site, extending the session timer. It will persist the same session ID as long as the expiry timer is active. Once the object expires, it will set a new session ID. Local Storage doesn't have a native method to manage "expiry" so we have to artificially manage it.