Love the dimensions of this tank! As you know, go slow and steady all you'll be okay. What rock are you using now? PNS substrate sauce is good to start as it adds some PO4 to get the bacteria thriving and you don't starve anything.
Got rocks off a forum member, so I can’t remember specifics. It was bleached, rinsed and air dried. Then soaked in salt for maybe 3 months while I kept making excuses about tending to the tank.
might be wrong but I don’t expect a spike because as I switched out the rock, there was always about 50lbs that’s still inside the sump. Plus I added Dr Tims anyway. When I set up the tank from day 1, I added dr Tim’s one day, and two days later added my first 2 fish.