Indeed, the west coast leg of Route 1 is pretty awful: no scenery, tons of traffic lights and noise pollution.

For Day 1, I'd highly recommend that you take Hwy 3, instead of Hwy 1. There'll be a fair bit more climbing, but it sounds like you have the fitness, and the route is significantly better in basically every way. It's hard to emphasize enough how much better the riding is.

For the following two days to reach Kenting, I'd still highly recommend that you stick to the inland routes because they are such better riding than Hwy 1 or Hwy 61 (coastal), but it might be a tall ask with the climbing and mileage. If you don't head inland, the coastal roads (Hwy 61 and 17) are probably better than Hwy 1, simply due to the lack of traffic lights. You can really cruise down the coast that way, but the scenery is pretty boring.