I wouldn't go to Bali for beaches, and I'm not at all a fan of Phuket (Khao Lak is ok, but I don't feel a strong urge to go to Thailand in general). Langkawi is great, but the Datai is crazy pricey, and the Andaman regrettably burned down (was insane value on BRG). I don't think the beaches in Desaru and Penang are anything special. Kota Kinabalu town stinks, but there are decent beach options nearby. I spent a few very pleasant and quiet nights at Hyatt Kuantan, but it's not much of a nightlife destination. I don't drive in MY/TH/ID, but outside of cities MY is fine, arguably better not to be dependant on crazy bus or Grab drivers.

Have you considered Phu Quoc? Beaches can be great or awful depending on the weather/currents, but there are some spectacular and reasonable hotels there (e.g., IC, Regent). Scoot is about to start flights SIN-PQC.