just as I thought I was done, typing revealed one last(?) change!

Double tapping on the number layer was
a clever way to allow keying symbols that would have otherwise required
either, shifting to the symbol layer or
adding a **Shift** modifier to the number layer.

Alternating layers is disruptive to typing flow. And, replacing one of
the existing modifier assignments with the **Shift** key, given the
modifier consistency across layers, is also undesirable. But there is
a downside to double tapping on the number layer, as clever as it is:
repeating digits is not that uncommon and efficient data entry easily
triggers unintentional transformation of repeating digits to their
corresponding double tap
symbols.

The solution? Another key assigned togglable layer was created! Leaving the existing modifiers untouched, the left pinkie toggles the corresponding symbol layer as defined below..

If the **Symbol** layer key had simply been defined as a **Shift** key
instead, all of the shift symbols for the numbers **1** to **9** would
have been easily available from the number layer.

Choosing instead to map a corresponding symbol layer, however, allows
adding the **Space**, **Comma**, **Semi-colon**, **Colon** and **Bar**
characters for added convenience in entering lists of numbers. Of
particular note, the **Space** key is not otherwise available to the
number layer because the left thumb is already occupied toggling the
layer itself, the **Space** key being assigned to right of the
**Number** layer key as can be seen here.

This last change preserves what the original double tapping design provided, with even a few beneficial additions. This could be the final firmware flash..