Yes, I have used those things and they are terrible. Some have the treads to big others the treads are the wrong pitch and some the holes are bigger than the bolts. If you put lots of bolts through your sandwich you should not have to use any connectors for the unistrut the plywood will act as the connector plates. But the angle plates that they sell, look as if they can be used by bolting through the unistrut. I think the unistrut will be about the same weight as the 2x3’s. I assume you are using the 2x3’s as cross members right? What type of axel are you going to use, leaf spring or torsion? The torsion is the easiest to use. You could get a piece of rectangle tubing and bolt that to the side lengths of unistrut then bolt the axel mounting plat to that through the floor. The tongue can be bolted to two lengths of angle iron and the angle iron bolted to the floor. That is what Cary-on trailers does. Use grad 8 bolts for that only two ½” through the tongue should work. Your floor plywood should be treated with rot-doctor before installing both sides for the roadside piece. Looks as if you have a good plan Just take lots of pictures.
Chuck