Hey Mike @MikeFinity,
it’s not only the risk that it tears, it’s also the stability during use (runout, chatter) that cannot compare. Milling motor mounts are definitely not made of PETG, even when reinforced with carbon fibers, and there are good reasons for that.
If also doubt that a Bosch 1617 router is suitable for a CNC. If you want to switch to using a CNC router, I would not save on the milling motor of all things. The reason that hobbyist cnc machine manfacturers offer machines for use with a hand router at all is that you already have one and so they don’t have to include the milling motor into the price 
But if you design a good CNC machine, even for hobbyists, I would use either a real induction motor like used in the industry (also called “spindle”) or better what I would say is best milling motor solution for hobbyists and semiprofessionals. That’s what is used in cnc routers in Europe (example motor in action, rebranded Mafell milling motor). Unfortunately the feature request for such a motor mount has not led to anything yet, even if there is a choice of reliable motors of this type made specifically for cnc machines available.
Also if I would replace the Onefinity’s stock Z assembly, I would never copy the strange Onefinity Z assembly design but mount a universal Z axis assembly, which allows
-
to protrude beyond lower end of Z assembly which elimininates “my bit cannot reach my wasteboard” issue, and
-
can take your individual specific motor mount because it has a flat surface with holes. Milling motor mounts are usually offered with the milling motor!
Welcome to the forum!
PS: You’re not mad at me Dave @tgunr tgunr, are you? I’ve also found that if you buy a 3D printer, at some point you can’t avoid contributing to plastic waste. It’s inevitable.