Wet season is May-October and it doesn't just switch over on May 1st. The weather is usually pretty good at this time but very hot (high 30's+) and often a short heavy thunderstorm in the afternoon or evening.
I have been going in May for 5 years now (booked again for next year). I feel it is a good time to go, only one year was there a couple of days of pretty consistent rain but apart from that just an occasional heavy downpour, usually in afternoon which passes very quickly. Hasn’t rained every day. Cheers
This chart https://www.google.co.uk/search?tbm=isch&sa=1&ei=o2OjW4awJszIgAbCsK_wCA&q=phang+nga+annual+rainfall+&oq=phang+nga+annual+rainfall+&gs_l=img.3...10135.24088.0.255184.108.40.206.0.0.0.144.1350.7j7.14.0....0...1c.1.64.img..1.0.0....0.iQx97loGjak#imgrc=UWvfZxK7YPj9TM: is for Phuket so Khao Lak should be similar. Not sure why May sticks out so much compared to June and July. Trying to stick to April may be better or June but it cab be so variable. I spent June in Trang one year and the weather was awful and cut our losses and headed to Hua Hin. On the day we left it was perfect and Hua Hin had no rain at all.
We went this year on the 27th March for 2 weeks and the weather was fabulous!
We have booked again for next year 2nd April and we're hoping the weather is equally as kind.
We go back end of April every year since 2013, hot, some days a tropical thunderstorm which lasts about half an hour. We will be back again after Easter next year. Also come up late October, similar but not as hot. Low season rates with usually brilliant weather in both months. We did Late May one year but too hot.