I'd probably have max 2000 lbs on a rack. In my last shop I used casters extensively because I had very limited room and needed the mobility. I had my (forgive me machining Gods) BP mill on heavy duty iron casters, not easy to move, but it would move. It's all about the diameter of the wheel. Something 2000 lbs on a smooth concrete floor you'd want minimum 3" diameter, probably 4".
Here are some examples of the idea of the bolt on casters/lift
https://www.howdoimovemyshelving.com/products
And some cheap Amazon items that are sort of the right idea.
https://www.amazon.com/TOOLGUY-REPUB...07MJTC4FK?th=1
I'd just make something myself, but the logistics aren't working out at the moment. Was hoping to find something so I could get a couple of racks moved in and mobile so I could start making the things I need, and not have to completely unload them and disassemble to put wheels on them later.
If I was brave I'd just pick a spot for the lowest beams and weld them to the uprights, then I could use pallet forks on the skid steer to move the whole thing, even loaded (carefully).
The example shown in the top link end up over $1000 for a set of four "jacks" and attachment adapters.