Exactly 6 months later (what a coincidence), I have answered this question. There was indeed a word to fill in the blank.
Backstory: I’ve been making more and more mind-blowing observations over the past few weeks about how useful ChatGPT is as a language learning tool. I realize some people have misgivings about using LLMs, but my experiences, to me, speak for themselves. And this is the latest one. I remembered this unanswered question from long ago, and decide to give ChatGPT a chance:
tl;dr the word is 促音便 (sokuonbin), and it differs from the suggested word above (促音化 or sokuonka) in that it only applies to the transformation if there are specific conditions surrounding it. So, sokuonka is the general transformation itself; sokuonbin is one specific instance of the transformation.
There are multiple references in the thread linked above. Two are: