A good workaround finally allows me to use the Cardputer uLisp Machine, a lovely card-sized microcontroller system that runs uLisp. If any Emacs wizard has any advice on how to fix the remaining minor echo issue mentioned in the post, I'm interested.
https://journal.paoloamoroso.com/paoloamoroso/making-ulisp-more-usable-on-the-cardputer