It looks like an interesting project.

If you made the frame with chromoly, you could probably use 1/16" tubing, and cut the weight of the main frame in half. Also consider 3 dimensional "truss" designs.

200 to 300 lbs will be heavy!!!

I regularly tow a trailer, and can certainly feel it drag once I reach 200+ lbs. Hit 500 lbs, and the hills are killers. But, even riding on flat land is hard with heavy loads. Even road crowns can be a pain.

And, yes, the weight of my trailer did add up quickly when I was building it... It doesn't take much for just the bike or trailer to get heavy.

Have you considered provisions for motor assist?