It also reminds me of a similar option, the Jupiter Ace [1] which ran FORTH natively. The co-founders worked on the speccy and certainly borrowed some design aspects from the ZX81 (had a better keyboard though - ha!)

The manual was great for its time, a little dated now but still a decent read [2]. It was reprinted in 2018 and still available [3].

[1] https://en.wikipedia.org/wiki/Jupiter_Ace

[2] https://archive.org/details/vickers_steven_jupiter_ace_4000_...

[3] https://www.amazon.com/Jupiter-ACE-Manual-Anniversary-Progra...