I would recommend simply using 6 load cells and 6 HX711 boards. My guess is, compared to the load cell cost everything else is very small.
It looks like your silo is about 12 feet high (rough guess based on the ladder). I would not consider adding a platform just to make the measurements easier.
The really problem I foresee is connecting everything up and surviving:
- ESD
- Lightning strikes in the area
- etc