Did you ever have any luck finding help with your GSA SER campaigns? I've been having a lot of issues with ranking too.
I'm starting to think that it has to do with other metrics like Moz/Majestic with their Citation Flow(CF) and Trust Flow(TF). They seem to suggest that Google uses metrics similar to those, where CF is the power of your site in terms of the amount of links it has, and then TF shows how reputable those actual links are. So if you build a backlink on a webpage that has a high CF and a low TF, then it looks spammy to Google, and that spammyness is passed along to your website. You'd usually want your TF/CF metrics to be close to each other. Usually the TF is a little bit lower than CF, but you don't want it too much lower.
I'm trying to seek help too. What I ideally think would be a good solution is... checking each URL to make sure it has a good TF before building backlinks on it. But this would require a tool to check majestic metrics in bulk. There is a tool for doing this on Majestic, but their subscriptions are so damn expensive just for me to "try" and see if it helps me rank a webpage/website.
Anyways, maybe if you could, let me know if you find out anything interesting on your efforts :)