Thakhek has the advantage of not being ridiculously hot and humid. There are some fun 5.10 climbs there, plenty to last you a week, although the rock tends to be sharper and more brittle on the low angle stuff. You'll want to make reservations at the Green Climbers Home beforehand, since they always fill up in the peak season.
Chiang Mai (Crazy Horse) is a really small area, but if you've only got a week you will maybe have enough to climb. The weather is drier and a little cooler than Krabi, but not as nice as Laos. Chiang Mai is a pretty fun city with a lot of good food options and stuff. Jira's Homestay is walking distance to the crag and a great place to stay.
You've already been to Railay, so you know what to expect there. Personally I'd maybe just go to Chiang Mai if you've only got a week (assuming you haven't been before) The logistics are easy for getting there and it's got the right amount of climbing for a one week trip. Go to Thakhek when you've got more time and are maybe climbing a little harder so you can make the most of it.