Understood - that image was of a ER11 collet on a 150mm spindle - useful for home-builds but I've since realised that you can easily buy similar with a much more usable clamp onto the spindle motor that you have. Pretend I didn't say anything on that side.

You're right about the V-Cutters, but the one I prefer is a 0.3mm diameter straight mill - so the cutter is about 2mm long before that bells out into a more conventional 1/8" shank - if you tried one of these with any runout the cutter would snap pretty quickly. V-Cutters are okay but they suffer if you have a warped PCB substrate. Isolation routing is not the easiest thing to get right, particularly if on a weak machine. You've got some fun ahead :)