Looks very well thought out and functional.

I made a couple of small wooden things similar to that to put under the back end of my kayak or canoe, so I can lift up the front and roll it on pavement or grass. The smallest one is only used on pavement, I used in-line skate wheels. To avoid having wheel alignment problems I used a single axle shaft of aluminum rod, and small clip pins to hold the wheels on. But the single axle would not allow you to remove the wheels to reduce volume like your design. In my case, making it more compact was unimportant.

Very good job.