Fixed the issue. PEBKAC! Hostname was typed slightly wrong for the Proxy , and my eyes must be getting worse.
Used KB7848 to check proxy match string in config. Restarted Proxy service and clients started connecting correctly right away.
https://support.eset.com/en/kb7848-write-a-proxymatch-expression-for-configuration-of-apache-http-proxy-with-eset-protect