Oh....a mill....I was wondering what it actually was before seeing the photo blown up bigger.
I think you've done a marvelous job under the circumstances.......backyard engineering using the utmost ingenuity is the very essence of inventiveness.
I have deep respect for anyone who can apply logic and put parts together to make them work despite many setbacks and re-visions of the build as it goes along......the end justifies the means in all case without exception.
One thing I think you will find a bother.......the spindle is only really suited for high speed carving work as the limited torque at low revs (for steel milling) could damage the VFD as it has to pile on the amps to put out the torque.
That would be the only area I would revise if you intend to do milling work......it is a mill after all not a CNC router that is more suited to a high speed spindle.
In that, I would consider a separate spindle and bearing set-up with a belt drive motor....possibly a 3 phase 1 HP type with a VFD to get the low rpm speed range and low down torque........VFD's can work on single phase electric supply and put out variable 3 phase to drive any 3 phase motor.......I have such a set-up on my small lathe....1/3 HP capacity.
Ian.