Ultimately it should be down to the ATO and its employees/contractors to establish a working environment that is efficient and comfortable otherwise you just won't hang on to good employees. Good TKIs are hard to come by.
Never a truer word said. I'm sure there was historical guidance that also recommended how many subjects a TKI should be expected to teach since developing the curriculum knowledge to the depth required to teach is considerably harder than the level required to pass the exams. I was a TKI for a short while and would return to it if I could not find flying work. However the pressure to learn and teach subjects as a new instructor with no preparation time allowed was something I found particularly challenging. I found that having classroom time of >18 hours meant I was having to work long hours to prep. On average it would take at least 3 hours prep per 1 hour in the class, sometimes longer when I was having to produce most of my material from scratch. Plus there was additional 1-1 time with students who were struggling with some sections.