Colemak folks have been using similar things and beyond for years. They are so sophisticated in the endless search for the perfect layout, it’s sad to see people unaware of existing solutions.

  * Is Extend "just another layer"?

  Extend, at its core, is a way of bringing navigation/editing to the home position. To do that really well you have to be able to chord modifiers and nav/edit keys easily from the home position. If you can do that with a layer, I'll consider it an Extend-type layer. Many laptop Fn layers, for instance, have only nav/edit and not accessible modifiers so they fail to meet my definition.
  I also have multimedia, system and mousing keys in my basic Extend layer. There may be other layers too, such as a NumPad layer and string/symbol/command layers.
  An Extend key is a layer modifier that lets you select these layers. You may have both Hold-Extend where you hold the Ext key like a Shift key, and Tap-Extend where you tap it like a dead key. Tap-Extend is best for one-shot stuff like strings/symbols, whereas Hold-Extend is brilliant for nav/edit and NumPad layers.
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  * ... Why?

  > Living without Extend at this point would seem like some kind of perverse hell imposed as punishment for terrible crimes committed in numerous past lives.
  ~ SteveP, creator of the Colemak-DH mod
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https://dreymar.colemak.org/layers-extend.html