Found a solution for formatting the .txt file.
TLDR: first line of the text file should contain a string of "{} {}".format(VOCAB_SIZE, NDIMS).
Sources for solution:
- How to use two .txt files one with vectors the other with words
- https://stackoverflow.com/questions/27324292/convert-word2vec-bin-file-to-text/29251943#29251943
- https://github.com/explosion/spaCy/issues/2298#issuecomment-387091526
Thanks for your help Ines!
Darren