The 300kg might be a challenge. It seems most ali express load cells are aimed at light weights, or weighing humans. Here are some for lighter weights: https://www.aliexpress.com/item/1kg-5kg-10kg-small-range-weighing-pressure-sensor-load-cell-set-weight-sensor-set-sensor-HX711AD/32765498523.html Here is one for 200kg: https://www.aliexpress.com/item/New-Arrival-1PCX-200kg-electronic-platform-scale-load-cell-pressure-balanced-cantilever-load-weight-sensor-New/32703877639.html If you can't find one that does 300kg, you could use a lever mechanism to scale the force appropriately. I also saw some adds for custom load cells, but that is more expensive.

Look up isokinetic physical therapy machines. They use stain gauges or load cells depending on the foce you want to measure. Expensive, but if you are studying strength of a joint through a range of motion, it might be what you want. Might be able to use one at a PT or university sports dept.