i can’t really be of much help, but if was thinking out loud:

• I don’t think belts can handle the cutting forces, they will also stretch which will (maybe) ruin the part, create chatter, snap bits etc.
• the old standard for small milling machines is that you move the work, so you use an XY table, like you find on the taig and sherline. if you went this route, know that they make ready made xy tables, you might find them in machinery shops. that might make things cheaper.
• why not make a voron cascade? at least you know it has been thoroughly tested. you only need one ballscrew for each axis.