Thank you so much all of you guys and girls for your advises on this "situation" 🙏 They, for sure, be usefull for the trips I plan to make in south Asia next year :)
For that (first) one, I certenly won't play with the devil (and "pay my peace") by asking for a tourist visa.
I called Lufthensa today. They were very clear : "Sir, you need a tourist visa for a 6 weeks trip" .... they did not seem to welcome the idea of ​​presenting them with a "run" ticket outside Thailand :/
I'll will flight to Bangkok from Francfort btw ... do you think a one-way Bangkok / Siem Reap is enough ? No need a return ticket ... in case the company knows the "trick" ? (I become paranoid with these visa stories)