I've done something similar, to what you're doing in the past. I took all the rock I had received from a 10 year old operational tank (covered in hair algae). The entire tank became an algae refugium when I set it up. Was nasty. I took the rock, washed it in muriatic acid (ate away the first 1/8 - 1/4" of the rock and removed the phosphates). Then, washed it in bleach for a couple days. Then moved it to totes of fresh water. I would keep it in a stagnant tote of fresh water for about 5 days, then move it to a new tote of fresh clean water. Dump the old water, clean the old tote thoroughly then reuse 5 days later with new fresh RODI water. I did this weekly for about 4-6 weeks. Then I replaced the fresh water with salt water, a powerhead and small heater. I wated 5 days then did 100% changeout of the salt water. And let it sit again. That's when I started testing amonia and phosphates. I still had both after all this. I continued the 100% water changes for 6 weeks. When phosphates finally routinely tested zero, I placed a mesh bag of raw shrimp in the tote and waited another 5-6 days.
After that 5-6 days there was zero shrimp left in the bag and started to have ammonia. Phosphates remained minimal during this time. After another 3 weeks went by I placed the rock back in the display (display was running with minimal fish, rock, but same sand bed.) I placed all the rocks and waited another month prior to adding more livestock. In total it probably took me around 4-5 months. But, I had brand new phosphate free cycled rock.
Yes, there'd be faster ways to cycle now, but, I was in no rush and the rock had so much phosphates bound up in it that no matter how often I cleaned them, scrubbed them, within 10 days, I'd have basketballs sized balls of hair algae just from hand pulling. It was either wash it or buy new rock. A $30 in totes and $5 bottle of bleach and I was home free 5 months later. It took a year from when I set it up, but that 125g display was finally looking amazing.
In between this I had tried everything, lights out, water changes, more and more snails, a home made turf scrubber (that I never got to work), vodka / carbon dosing, nothing worked. I hope microbacter clean works for you!
Then my house burned down and lost the entire tank in the fire.