Building a No-Weld Trailer Chassis
In my analysis your design is not going to be adequate. Here are the main points:


Box steel with just 3mm thick walls can deform when squeezed under bolting pressure or when subjected to torsional pressure when bolted through the box.
Carriage bolts with the square tang under the head are meant to be used with wood. Unless you intend use dozens of hours filing round holes to square the carriage bolts are not suitable. If you used a hack to make the round hole big enough diameter to accommodate the square tang then you have lost the precise alignment that you mention in your question.
For a bolted construction I see a severe shortage of triangulation designed into structure to keep it square.
The cantilevered cross members should have some type of edge interconnection to define the outer perimeter of the frame. The need for this would become much more obvious as you address building the next levels of the box trailer.


With the time and materials that you would spend building this you would be better off just considering purchasing an electric arc welder and doing some practice welding on scraps of steel to brush up your technique to weld this frame construction yourself.