Best time to visit is October to April? January to June is burning season, have you been to Laos during peak burning season around Feb to April? Most people say its the worst time to go, not the best?

Also what do you think of Chiang Mai in Thailand during March or April?

Yup, that's a big problem all over Southeast Asia. Worst time? That depends on what you prioritize. I personally prefer clear days and cooler weather (for photography) rather than hot, humid, and rainy, and that period is optimum, if you don't factor in air quality.

If you really want to avoid the burning season, October - December is your best bet but even then, it's not guaranteed. At least the weather is cooler though which will be great for outdoor activities.

I wouldn't recommend visiting Chiang Mai in March and April. The weather is getting more humid and the air quality is still terrible. Stick to November and December and both should be more manageable.

I hope this helps.