Mystery Penalty

[execubob]

I have a client site that seems to have been penalized with a huge drop in rankings site wide for all keywords. I would like to hire an expert that could figure out what the issue may be.
HOWEVER, I've done all the standard audit items, including submitting a huge disavow file. Nothing is reported in GSC and the site is built out in pure HTML, so not a WordPress issue.  I don't need a basic audit, I need someone with expertise in finding a possible penalty.