These days cheap PBN bulk posts arent delivering as they used to. Domains get deindexed too often and you can't stay on top of it. I would go with SAPE for Churn and Burn if you have a process for picking out the best links. You can throw cheap blog posts around week3-4 to boost things even further before you start tanking.
Absolutely true that cheap pbn posts no longer do much, at least not when used as part of method 2 described above. I don't know why, but your explanation makes sense.