There actually is a macOS app called "Sync Folders Pro" on the Mac App Store that can be set to Auto Sync two folders at a user defined update rate:
https://www.greenworldsoft.com/sync-folders-pro-help.php#Automatic-synchronization
Sync Folders Pro has different sync modes. You can for example ask it to replace existing files, or not delete existing files when syncing, etc.
There is also a "Real Tme Sync Mode":
https://www.greenworldsoft.com/sync-folders-pro-help.php
You can also script it to exclude file types (e.g., the JPEG thumbnails in the ASIAIR).
Chen