Welcome Tanaka-san! Looks like you’re going down a similar road to me when I bought my Nomad 883 Pro.
You might find my long adventure pushing the Nomad 883 Pro to its limits to be interesting.
+1. Not a fan of TiN, prefer ZrN or diamond.
Where was the power meter placed? Was this between the Nomad and the wall, or did you manage to connect it to the spindle itself? What style of meter is it? A clamp meter or one of those outlet measuring devices?
What kind of load is it? Are you increasing feed rate, width-of-cut or depth-of-cut? In my experience even with the drastically underpowered 883 Pro, power isn’t the thing that’s likely to cause behaviour like that, it’s torque.
On the Nomad 883 Pro, 6mm or 1/4" endmills worked fairly well for me, as they give you higher surface speed for the same RPM. You have to use a small width-of-cut/stepover though.
Also, don’t run at speeds as slow as 50mm/min. You can play with most of the feeds and speeds numbers but don’t play with feed-per-tooth too much. That, in my experience, is the one that’s most likely to cause problems if set incorrectly. Too high and it stresses the tool and breaks it, too low and you get rubbing. The manufacturer of an endmill should usually provide recommended feed-per-tooth for their particular endmills and it’s best to stick to them.
Also, what was your width of cut in all of these cases? You haven’t mentioned it so I have a feeling you might be doing 100% stepover, which would cause problems like you’re describing.