Simon 5 is coming soon!
Simon version 5 is currently in beta testing. It is a massive update, introducing a much-requested feature: syncing the Simon data between multiple Macs, plus a Simon Status service, a Link Checker filter, improved Find filter, iMessage and Speak Error notifiers, a new app icon, and much more.
Important: please note that Simon 5 requires a minimum of macOS 10.12 (Sierra), and will be a paid upgrade from Simon 4 after the beta. Purchasers since September 1, 2020 automatically get a version 5 license (that also works in previous versions).
Re: Best way to monitor a remote Apache error_log file
Yes, that'd work. Since you don't want to fetch the whole file, you'll have to provide a subset of it to Simon.
Another option could be to write a PHP-based page that extracts the tail of the file, rather than using a cron job, so it's always fresh and doesn't run unless needed.
You may be able to use a Script-based service to do the work remotely, too; perhaps SSH into the server and get the tail of the file.