My partner books a lot of travel for people who travel a lot, and while some of this is broadly true, I think the devil is in the details. They get a lot of preferences from people who arguably should only care about their time, but in reality also have comfort preferences.
When you say that shortest/fewest stopovers/shortest stops are all synonyms for efficiency, this is true, but also misses human factors. Some people prefer a single flight at nearly all costs, some people prefer to have shorter flights with stopovers even if the total time spent is slightly longer. Ask someone if they'd rather have a 45 minute stopover or a 1.5 hour stopover and I bet most would choose the latter, despite taking more time.
> time of day?? Easy, just add it to your request (works in sim too).
This also misses the fact that everything is about tradeoffs. I'd like a flight after 9am, the tool tells me that will be A$500, oh yeah, you know what I don't mind flying at 8:30am for A$200.
I think for a tool like this to work beyond a specific audience of optimisation nerds who only care about their time, it needs to treat everything as negotiable, and then present sensible alternatives outside of spec. Sacrifice time for lower stress, sacrificing seat preference for sooner flight, sacrificing airline preference based on price, sacrificing destination based on availability.
Being close to someone doing this job, and having travelled a fair bit of long haul over the last few years, I know that "efficiency" is very nuanced with personal preferences, that most people don't give even a fraction of the required information to be able to actually do this well and back and forth is necessary.
I wish you all the best of luck, if you can make it work, great!