Please can you explain how you are classifying small vs larger ones? I.e. What is the maximum size you can use a harmonic drive for and what prevents you from going larger with one?
I guess that for my needs, I could manage quite well with an indexing solution instead of a simultaneous 4 axis capability. My interest would be in milling an aluminum rifle chassis in one set-up. My main concern is accurate indexing so I could machine both sides of a stock or chassis with accurate inletting. I have seen plenty of low quality stocks with more material on one side than the other due to poor indexing.
I have been wondering if I might be better off with a decent manual rotary table (with a good break) and turning the part twice. As I don't have an ATC spindle yet, it's not like I could leave the machine unattended and if I have to be there for tool changes anyway, maybe manual indexing wouldn't add much burden.
A lot will depend on the availability of quality used parts to make a 4th axis with acceptable performance. It sounds like the cheap Chinese belt-drive 4th axis won't cut it (literally) for aluminum. I occasionally see used rotary axis from old VMCs going for less than $1,000 so it's either that or a harmonic drive.