But you still travel by car several times a year, so not putting you off that much then.
How many dead cyclists have you seen at the side of the road?
I've cycle toured in Thailand for ten weeks, but it was 25 years ago. Didn't feel particularly dangerous back then but we did avoid the busiest main roads.
Well, as for the journey, we don't have much option really - trains are very inconvenient for getting around in any comfort, and we need mobility when we get there. And we're able to choose times of day (and times of year) when the traffic is relatively light. And in our 4x4 I reckon we're a lot safer than on a bicycle - besides, we protect it with all the right Buddhas, amulets and lotus flowers
As for how many cyclists I see, I pretty much never see any cyclists at all outside quieter residential areas and country areas where bikes are common transport.
Traffic is much heavier now than 25 years ago, but your point about avoiding the busiest main roads is a good one. I think it's relatively safe to cycle around up-country areas, resorts, islands, but I'd never do any long-distance rides on main routes - like Bangkok to the South, as the OP suggests.