Large chalkboards like that used to be split into sections. Well, some of them.
If you have a cart like that you don't want the outer lower beam to be across. It's likely to twist off if you're not careful to stop it before you stand it down.
Just a box with wheels and a handle is a bit on the old side when it comes to inventions. We're talking thousands of years BC. The sack truck or whatever you call it is significantly more modern and would make sense as a new invention here.
If it's harder but heavier, I wonder if using less of it to the point it has the same weight would make it more or less durable than mythril. If it isn't more durable at that point, you can just use more mythril instead.