Milo, great questions!
CMM is a really interesting possibility, as is milling harder materials.
Taylor and I have been mostly focused on getting the DIYLILCNC built and putting together the documentation so that others can build it; our main build is also currently on display in a design exhibition, so we don’t get much access to it until January.
All this means that we haven’t had a ton of opportunities to really test the limits of what we can mill and what the tolerances are.
The end mill possibilities are pretty good, although the dremel is not the ideal tool for harder materials (There’s some more info about this in the conclusions section of the instructions). We were able to get a pretty good range of end mills from McMaster-Carr.
Let us know if you do end up building, we’d love to see pictures!