Castors can work, but you have to spend the $$ for good ones that have brakes and lock the swivel wheel as well. For most work they are fine, but if you have a vice on the castor end and are using a saw or a file, then the wiggling bench can be a bit annoying.
We had some BAC benches at work with fixed wheels at one end and legs at the other, they worked pretty well. Nice and solid, easy enough to move and didn't wobble when you were working on them. Backing them into a corner was a bit of a pain though, needed a bit of backing and filling to get them hard against the wall.
With the frame you have, you could get a pair of fixed castors, trim the mounting plate a bit and bolt the plate onto the side of the leg so that the wheel bracket is on its side, bolted to the side of the leg, but the wheel and leg sits flat on the ground at rest.
Then when you lift the other end, the leg lifts off the floor a couple of mm and you roll it where you need it, then it rests on the 4 legs for work.
Something like this won't break the bank and would prove proof of concept, but I'd go the really heavy duty option as it will be under a lot of stress outside its designed load paths.
https://www.bunnings.com.au/easyroll-50mm-40kg-load-grey-rubber-fixed-plate-castor_p3940309
A few other options, but the $$
https://www.machineryhouse.com.au/w935