I had a minor victory on my road to figuring out how to use my new CNC router.
I received the 0.5mm carbide end mills that I bought on Amazon and used one to successfully shorten a proto PCB that I need for my LoRaXes project. The mill has about 4 mm of cutting depth. The PCBs that I'm cutting are nominally around 1.6 mm thick. Not a very impressive looking cutter but it worked. I increased the spindle speed to 3000 RPM and reduced the feedrate to 80 mm/min and set the cutting depth to 0.15 mm per pass.
Here is the shortened PCB next to one that is uncut:
The cut edges are very smooth but you can see that they are not quite straight. Not sure if I can improve that with tweaking or if that's the inherent limitation of an inexpensive router and cheap end mills. Only time and experience will tell. Anyway, I'm pretty satisfied with the outcome. The tool did what I needed it to do.