Hi, I'm currently on a 5 year Elite visa and will be retiring before it runs out. Currently I enter and leave Thailand 8 times per year but once I finally retire it'll be a single fare with no return leg. This might be bleedin obvious but when that day arrives what do I put on the departure card?

Just put down whatever information you're sure off and leave the rest blank.

Just leave the flight number out. No problem at all - I do this every time.

You don't need to fill out the departure card at all until you depart.

I never fill in anything on the departure card until I actually depart. Immigration doesn't care about that half when you're arriving.