I read of someone else milling stainless steel in another cnc machine with similar rigidity. They were small parts and took a while but they came out ok.
As far as I remember the spindle used was just under 1kw. I’ve been looking into it a bit and I’ll probably try it myself too once the machine is all setup properly and I have some experience with other materials.
I don’t think it’s something to be milling regularly on these machines anyway. You’ll likely end up taking very shallow bites out of the steel which is going to be harsh on you’re tooling. They’ll wear out prematurely because you’ll only be using a small portion of the usable length.
Definitely something worth trying though, I’d love to see how you get on if you try it.