Bill Tindall, E.Tn.
Informative description. Suggestion- put the picture of the end at the beginning so to end result is shown before the steps. I found myself scrolling to last picture to understand previous steps.
Dado plane- Those lengthy steps to get a dado would be made easy with a dado plane. Here, the old ones are collectors items and expensive. Curious LV has not offed them. I'd buy one , cheap as I am. Cutting dadoes by machine is fast. That is the problem. Things can go astray fast.
Next time instead of cutting two templates for each size, cut one and a 10mm spacer. The spacer can be diddled with(AKA planed) to get it perfect. More practical than diddling with each 10mm longer template to get each exactly 10mm longer. Hang spacer on the wall and use it next time too.
Also curious your use of a secondary wood in this small piece, especially one that doesn't work well. My choice of secondary wood is driven by works well. I can't imagine significant money was saved by not using maple for the whole thing.
You have inspired me to use a mitered dovetail.