I don’t know if this helps at all, but we haven’t changed our website in Years (5+). There apparently was a shift in the Google Algorithm a few months back and our conversion/traffic went way down. (That one was a NON-Wordpress site. It wasn’t spammy, and there was no advertising on it. All we advertised was a service that we provided for a particular area. That is it. Like I said, it has been very steady and very consistent up until a few months ago, and then it hit a HUGE decline.
I have seen this before, with clients of ours too, and the Google shifts.
Sometimes they (Google) re-tweak the algorithm and things come back, but it can take a few months.
I realize that this may not ‘fix’ your issue (or mine), but hopefully it can provide a possible reason for the decline, some hope, and some piece of mind that what you have done may have had nothing to do with it.
Wish I had more to offer.