Let me propose a bold generalization based on observations of sites of all sizes wrestling with Google over the last 20 years, and to which there are certainly exceptions (this case may be one):
It’s always one thing.
Of course, there are always a multitude of improvements that can be made to nibble around the edges and get incremental improvements in various metrics. And a gradual change in traffic may have several simultaneous sources.
But when the effect is an identifiable precipitous drop in search traffic, almost all the time, it turns out there was a single reason, whether a change Google made or a change the site made.
This may be a totally unhelpful observation, and feel free to ignore it. But in these situations I’ve come to find it’s more fruitful to look for A Cause than to approach it as “a little here, a little there.”