There is not enough data for an accurate recommendation. In September, you can go to any coast and to any island in Malaysia: it's still a dry season in the east and already the end of a wet season in the west.

Question, how much do you want a romantic and secluded holiday? What exactly interests in traveling by car? Perhaps you should start with the cultural part of the program, and then relax on the beach? And the "sky-high" prices for hotels of the 4-5 * category are also a very stretchable concept, so I'll try to sound several different options.

In general, Malaysia has an excellent service and a high level of hotels. If you want a beautiful virgin nature, romance and seclusion, look at the Hotel Japamala on the island of Tioman. It's possible to drive by car on the island, it's quite big - but the question is what exactly do you want to see there.

Similarly with the island of Pangkor on the west coast. Nice secluded beach near the hotel Aryani Terengganu - and there is an opportunity to travel to two states. You can see the hotel Nexus Karambunai in Kota Kinabalu - a clean beach, a bit less romantic solitude, but there is also where to go by car and the sea park "Tunka Abdul Rahman" with fairy islands in transparent water - nearby.

And you can stay on the "classic" holiday in Malaysia - the island of Langkawi - a good combination of beaches and opportunities to travel around the island, there are enough sights. The hotel can choose any - what you like for prices, location and pictures. The island fascinates many tourists, but the prices are higher.

Good choice and a good trip!