I am not worried about the feasibility of what I am looking to do. It's nothing new or unique. Plenty of people are already doing it. I'm just not sure if anyone is doing it on a cheap Chinese 4th axis, or what the minimum investment requirements are for a semi diy build.
I am back to being as confused as when I started which is a fairly common issue for me when I ask for advice on forums on topics with no definative answer. Conflicting advice would be fine if I knew all of you and could just choose to listen to the most knowledgeable but I don't.
To me it reads: you want a harmonic drive. No you don't, that's the last thing you want. A cheap pulley system will
do the job. No it won't...well... not if you want the end result to resemble the actual product. What you really need is a worm gear system. No! Definitely not that! My uncle Dave once built a 4th axis from toe-nail clippings but I have no idea if it actually worked. Wait... it may not have been uncle Dave, it was aunt Susan and she never even had toe nails....
I am back to attempting to use common sense, deductive reasoning, reading between the lines, trial and error and trying to identify common themes from the advice.
Starting with common sense and common themes, it sounds to me as though any type of motor and reduction system could work well as long as I buy "good ones" and use a motor / reduction combo that provides the right amount of holding torque and.... I mount the components on a suitably robust frame with a well supported grip. Similarly, I bet any type of drive could fail if I buy a low quality version.
I did a bit more research and found a good number of examples of 4th axis builds using rotary tables. I also found some impressive projects using harmonic drives. There is a good selection of those on eBay used at affordable prices. If I can figure out how to connect one to a chuck, I could see it working well.