Mui ne can be dirty sometimes like after a big rain but that does not happen often in the windy season. Mostly quite clean, not crystal clear clean but acceptable. Main beach gets a dead chicken or 2 washing up sometimes. I like to go to different spots in Vietnam and some are pristine beautiful, loaded with fish, coral, endless deep blue water, ect.
There are rules to kites flagging lengths. If you flag to 2 front lines, the bar must travel not less than one width of the kite (length from trailing edge to leading edge) before it reaches the stopper ball.
If it flags to a single front line it must be not less than one kite span (length from wingtip to wingtip) before the bar reaches the stopper ball.
These rules apply to the largest kite in your quiver. No harm in having the bar go further.
Looks like you're not so bad with tools and plans yourself...
If you ever produce the fiberglass handles (or carbon fiber) I'm in. I was thinking of doing it myself with a simple wood mold release, even have all the materials, but It's on a distant back burner.
6-10 weeks a year is awesome especially working like you do.