RustyC

I shudder to think what Siem Reap must be like these days, but definitely go because it's only going to get more so. When they open the McDonald's on the road between the town and the temple complex the tipping point will be officially past. Dunno if they've still got enough room atop that viewing hill in late afternoon. One strategy is to zig while others zag (tour buses tend to go Bayon-Ta Phrom-main temple). It's considered disrespectful to take nude selfies in front of the carvings.

Phuket and Krabi are similarly overrun vs. 20 years or so ago, but you might be OK in November, and it's always possible to do what the Kardashians did and mostly coccoon at some 5* place. Lanta and the other "newer" islands represent a longing for the kind of place Phuket or Krabi or Samui used to be before the big crowds arrived.

It'll be a good trip and you'll probably want to come back, so it's best not to cram the itin too full. I tried to do that on my early trips.

The attached pic, BTW, is Maya Bay (made famous by the movie "The Beach") on a typical day in high season. Like I said, too many people vs. before.