The spindle collet might take 16mm cutters, but at those spindle speeds that's going to be for cutting wood/plastic!

My own steel-built router has the usual 6K-24K spindle. I've been cutting steel with it recently, using carbide cutters 2-4mm dia, 3-flute. Typically, I'm running a 4mm cutter in steel at 6K RPM, 125mm/min, 1mm depth of cut. That's based on manufacturer's data sheet, plus experimentation. The cutters are capable of doing more but the spindle runs out of torque at these low speeds and can't handle the loads that the cutters could. I'm not necessarily recommending these settings, but they work for me so that might give you an idea of what you might be able to do. No way could I use a 16mm cutter in metal, although the spindle is capable of using something like that cutting wood. A lot of my cutting is also small fiddly bits, where a machine tuned more for acceleration than max cutting speed is useful. This link gives an idea of the art of the possible.

I also use Fusion 360 for design and CAM. Great tool, but especially with the CAM module, it takes a while to get used to the parameters that you need to set. However, now that I have been using it for a little while, actual toolpath generation is pretty quick.