The Esperanto stemmer seems to work well. The results look reasonable to me, several people have given generally positive feedback, and the questions and concerns people have had don’t indicate that the word groupings are so bad that they wouldn’t be useful, overall.
So, the next step is to create an Elasticsearch plugin based on the stemmer. This stemmer's framework is based on the Serbian stemmer we converted to a plugin before (T183015), so it is GPLv3.