Em .. everything depends.
I think most users missed the 200 mm D endmill, = 20 cm !!!, == 8 ", and carving wood 800 mm in D. !!!

You get huge forces from the leverage arm, and rigidity is always everything.

I suspect you need 4-8x the section sizes you are using and a box section spindle holder.
For the 200 mm endmills in wood at 800 mm D on blank, indicating about 400+200 == 600 mm free length (++) on the spindle ram/holder.