Closing the loop on my previous question ...

Lufthansa was nice enough to check our bags all the way to Koh Samui, even though one of my legs was on Bangkok Airways. The agent could not print our boarding passes though, which was expected, but I was able to print these at the domestic transfer desk at BKK with not much issue.

Furthermore, 2 hours was sufficient for this international to domestic transfer. I still had about 20 minutes to relax at the Bangkok Airways lounge after about 35 minutes of walking.