wincache not offically supported beyond moodle 3, we are sticking with just php opcache.
otherwise your hardware, software and settings look great. we have 100-400 concurrent users , windows 2008 and lower specs and its working really well on Moodle 3.3, php 7.1. we have the web and database on separate servers.
Disabling plugins one at a time, or disabling them all and adding them back one at a time is a good strategy to find a potential culprit as well, most of my issues over the years have been plugin related.
f12 and Network traffic tab in a browser is another easy way to find bottlenecks.