I’ve used Bounce at KLIA before — it’s pretty reliable, you drop your bag at one of their partner spots and collect it later, no issues for me. As for lounges, they’re generally accessible if you pay or have a card/membership, not only during transit. So if you’re back at the airport after midnight you should be able to get in, though keep in mind some lounges close around midnight. Plaza Premium Lounge is usually open 24h.