July Core Update - massive drop from #1 to #15 position

I work for a local web develepment agency and our SERP possition droped from a 15 YEAR OLD avarge being around the 1. and 2. spot to 15. place and has been there for past 3 weeks now. We have a high high quality backlink profile from having a signiture with a link in all of our sites/stores we produced over the years. High site speed, domain age, and good qualty content. What can we do? Managment is in panic mode We tried everything.

Edit: Same thing happend to our direct competitors(the bigger firms all droped to 10th position or lower and on top are now just some freelancers with new domains with 50 or less backlinks while sites with 250k or more (legit) backlinks dropped massivly