Actually, I'm not the guy to ask about feeds and speeds for steel. I believe the program is telling you the correct power. I'm guessing that for that small bit though, the RPM is too low, even for steel.

A mill like a bridgeport might have 2HP and can use much larger bits than you are planning.

Try using some 6mm or 10mm bits with faster speeds and feeds and see what the program tells you.

I'm guessing your spindle will not be a problem to get you to adequate torques with small bits like you are planning. It's getting the machine stiff. That program assumes that your machine is a heavy duty, rock solid machine, so what you are able to achieve in practice is still undetermined.

I've been thinking a bit more about this. I'm wondering if you should just go ahead and replace all the existing round rails to square ones, redesign your gantry side supports to be more triangular and move the bearing placement a bit, bolt the aluminum sheet on and add ribs like we discussed before, and make the Z out of a square steel tube, epoxy granite in every hollow. That would be way less work than a new machine. I recommend looking closely at the builds from linux_fan and ThomConcept before you decide on upgrades or a new machine. Both have ribbed structures and epoxy granite filling, but one gantry is aluminum, and one is steel.

That would make your modifications to be more like the two examples I posted. I've seen many examples of people who tried and did not do so well, but those two examples are the rare cases of people who made machines that can cut aluminum, and even steel it would appear, quite well. Neither one uses T slot, but also, I don't think many people use two pieces of T slot with plates on front and back and ribs inside and all voids filled with epoxy granite.

Of course, what you decide is up to you. If you like this machine the way it is, you could still spend some time designing a new one.

I agree with Ger21 that I have also read several posts on here from people who have found the round rails to be the weakest part. If you do decide to upgrade the machine you have, you could plan it out to do every change, but then see how it does after each one.