I did a simulation with your parameters received 5,67Ohms, for 8pcs of 3m copper bound steel rods, in hollow square configuration, after adding another cross-connection copper wire (all wire with 10mm dia., around 150mm2).
I tried double square configuration, keep squares 3m distance one from another (square in square), add more rods, total 15. Still, resistance too high : 5.29Ohms.

You need to improve earth with the active electrodes (with salt), etc, which is beyond our calc, and should be done by supplier of such electrodes.

Cheers