Nothing wrong with a road trip as it is a nice way to see rural Malaysia. It can be slow and not very nice though (on the west coast) but I think the east coast is still good. I drove from Kota Bahru to JB 7 years ago and it was good for the most part though lots of palm tree plantations in the southeast corner of Johore. Roads weren't too busy back then except around the major towns/cities. FWIW, Sixt and Avis offer O/W KUL <=> JHB rates (I've booked for each for early next year, but there's a limit on the number of cars they'll allow so it's been showing sold out for my dates for the past few weeks).

Desaru is a beach possibility but waves can be rough (Cherating is sheltered) and subject to shipping oil polllution.

Just read recent reviews of property. Malaysian hotels seem to fall apart fast. I suspect that part of the issue is using imported fixtures of which replacements are not easily obtainable and then there is competency of the maintenance staff (and to be fair, probably their lackof access to parts and tools).