I ended up pretty much going through all of that when teaching the kids.
Scouted around for an XXS Seat harness couldn't get them small enough here at the time found one in the US
Ben Wilson below the bar depower then later on the small Airush bar.
Didn't have the option for smaller chicken loops at the time so trimmed some back, was easy enough with the BW Bars.

I'm a bit T-Rex arms. Adjustable trim strap (height on the bar) was a useful crutch but in hindsight I should have just learned proper edging technique.

All your tips are useful Christian, but I think the stopper ball presents a problem without a kite with short sheeting range (the North Rebels for example were good because they had very on/off power) then you do tend to find the power overwhelming on the beach because you can't sheet out far enough - even though the power on the water is fine.

The unpopular advice would be to encourage an extra lesson to sort out that edging technique and show better kite control techniques - e.g. fly the kite low when very powered rather than high. Whilst you 'can' figure out all this stuff over time, I reckon I wasted so many sessions trying to figure it out because the kite schools seemed secretive over what was in their intermediate lessons so it wasn't clear what I would actually get out of the next stage.