I do use an Elgato stream deck for entering Unicode symbols, and I think what's really needed in this space is a cross between a stream deck and the Mathpad: programmable keys with LCD key caps, and some shift keys to increase the number of available symbols. Then it could support configurable symbol sets.
1) Over 100 symbols immediately accessible
2) Single-hand use
3) Small size
The elgato stream deck meets some of these requirements, but it would not have been possible to support the amount of symbols I wanted.