This time I didn't need to start from scratch, since there already is an awesome layout existing for this one: the K5/K5m layout by Brett Anthony (aka thedood). It is designed to edit K5 patches (so-called Singles).
https://mididesigner.com/qa/7479/layout-kawai-k5-k5m?show=7479#q7479
So I just needed to rearrange things, add some missing functions and involve some newer functionality such as the ability to receive existing patches from the synth. Nevertheless Kawai gave me a really hard time again to understand their obscure MIDI sysex format.
So now at the top there are 4 green buttons to switch between Edit (the main K5 edit page on the left plus DFF, LFO and Key Scale on the right), Source 1, Source 2 and Both. In all I tried to arrange functions as much following the K5 screens as possible.
Each source starts with the DFG on the left and the heart of the synth, the DHG, on the right. Here the biggest limitation of MIDI control becomes obvious, since changing levels of selected harmonics following the filter settings (Range from-to, All-Live-Die, All-Odd-Even-Octaves-Fifths) is a relative change and cannot be controlled via MIDI. Also I need to see the current spectrum on the K5 display. So I would do the DHG settings still on the K5, while operating everything else from the iPad. This includes the DHG envelopes, the filter and the amp and this way I work much faster, fully concentrating on my musical idea.
I also added a Streambyter input rule to enable the layout to understand incoming patch dumps from the synth.
Finally I did fix a number of bugs, which occurred during testing over the past few weeks. Everything should be fine, but if you still find a problem, please let me know.