Thanks. I’m sure I will need plenty of help. 
I was looking for a way to show the flex I see. But somehow I can’t see it in a video or it looks much worse than it really is.
I needed a bit more control. This is what I came up with.
I put a weight (2.5 kg) on a piece of rope and hang it from the end of the end mill over the edge of the table.
I put the dial indicator on 0 without load and it moves 0.23 mm with the 2.5 kg load.
In the build instructions for the Burly MPCNC I remember reading something about belt tension and cutting forces. That was about cutting alu and according to Ryan it takes about 2.4lbs of force to cut aluminum
Now I found some info about side load in an old thread here. Maximum side load
In that thread Ryan says most MPCNC’s (Burly or Primo?) can do 1.9 kg.
That gave me another idea. I put the same weight and rope on a piece of MDF and see what happens when I let that weight pull the MDF against the end mill. Very little cutting and a burning smell. I have to push hard to get it cutting. That was with 6 mm DOC. With 2 mm it will cut and with just 1 mm DOC it cuts cleanly and quick.
To me that sounds like the cutting forces are way to high. And the only thing I can think of is that I have crappy and mills.