All is well and starting to breath a bit easier after fighting the crazy issues Ive had with the startup of this tank.
Tank is going through the uglies now. Diatoms/cyano and start of some gha. All good signs that the cycle is progressing normally with the new rock/sand. The tank no longer looks sterile either. Water chemistry is also now rock solid. Alkalinity no longer drops and has been holding in the 8s for over a month. So quite confident that the rock/sand originally used was the root cause of the issue.
Best indicator that the tank is on the right track now is that the corals are doing fine. Previously they would slowly shrink/die. I believe the bacteria colonies were likely composed of ureolytic bacteria that were in the original rock/sand and were stripping the water of all nutrients and the corals were starving. The wildly fluctuating alkalinity/calcium as well wasn't helping.