I think a servo would be overkill for a toolchanger!
Small stepper and driver would be far cheaper, and just as effective, as you don't need high acceleration or speed, unless you want to spend your life putting the tools back in the disc...

The mishmash of collets I seem to be gathering is what will probably push me to either buying a batch of BT35 ER toolholders, or swapping the spindle. When I need to swap tools, I seem to spend longer hunting down collets, or having to swap tools around to toolholder/collet combinations that will work for what I'm trying to do.
I seem to be managing fine just now, but I've not been using my Triac that much. I do have some new products on the sketchpad though, which are going to require a re-think of tooling, as I don't want to lose time having to swap tools between holders.