Tricky problem. The Haimer probe is very nice, but not exactly small - are you sure it would fit when attached to another contraption that further extends the z-height? Just curious.
I’m using 12 mm tools on a Shapeoko, but that’s with an AMB 1050 FME-P with ER16 chuck, for which there are (out-of-spec) collets. Runout with that is worse than with the standard collet sizes, but probably a lot better than an extension that appears to be held with grub screws. Of course, that may not matter for your application.