I believe empty weight is approx 8000#, so I'd guess loaded with 3 horses, etc would not exceed 8500#, but that is only 2 times per year and travel is 1500mi each time with moderate weather.
Ah ..... Mmmm ....... Empty weight is 8000# and loaded is 8500#, so each horse weighs 500#? Not to mention equipment?
This doesn't sound right.
Also, how did you originally determine replacing every 3 to 4 years? That might be an indicator of severe operating conditions.
Plan of action?
You're 2 years into a 3 to 4 year replacement cycle. That cycle length seems to have worked in the past. The question you are posing is can you change to a longer cycle ?
Good news!! You have plenty of time to figure this out.
The next trip, measure your pressure build up:
Measure and record the pressure and ambient temperature immediately before starting out for a long haul. After an hour to an hour and a half, stop and measure the pressure and ambient temperature. You do NOT want to see more than 10% pressure buildup, excluding ambient temperature effects.
Ambient temperature effects: Pressure changes 2% for every 10°F in ambient temperature.
If you see more than 10% pressure build up, you need to do something - increase load carrying capacity by either increasing inflation pressure or changing to a larger tire size.
If you see more than 15%, you need to take action
IMMEDIATELY! Slowing down is the first step, followed by increasing tire load carrying capacity when you get a chance. We're talking within the week, not months!
You should repeatedly measure the pressure buildup to be sure you have a handle on it.