I agree with the tire change interval. I have had so many friends have trailer tire failures in recent years.
Not the proper way, but I change out one tire every year. Every 4 years they will have all been replaced. I haven't had a tire failure yet with my trailers in 15 years of doing this.
I have to mention this though. I had a 1976 holiday trailer in the early 2000's. The tires looked like they were original G78's, and they were pretty much bald. I took that trailer down many gravel roads. I even had an axle break at the leaf spring, where the tire was leaning into the frame for a few miles. NEVER had a tire failure.
Just a testament of the quality of tires these days or perhaps I was just lucky?