How to sync a folder on two macs

Try the free Sync Folders app. It should do what you need. A much more powerful app for complex syncing with lots of control, scheduling, etc. is  ChronoSync. I use it for backing up several different folders to different external volumes.

You have all the necessary tools at your disposal using UNIX commands in the Terminal.app, but some folks who are not familiar with that should leave that well enough alone.

There are several Mac-friendly sync tools, which may in fact actually run those UNIX commands behind the scenes for you in a more user-friendly interface. Consider something like this, for example, or search for other Sync apps in the App Store for something similar.

https://www.econtechnologies.com/chronosync/overview.html

iCloud is absolutely not the answer if you don't want to sync large amounts of data over the network, that is about just as bad a solution as restoring from a Time Machine backup over WiFi Time Capsule. There are lots of other cloud-based services, but why wait to load your data on some other machine over the internet, when you can just copy your own personal and private files directly from one computer to the other on your own fast private home network?

Are you open to any cloud service? Microsoft's OneDrive is good, and more flexible than iCloud Drive when it comes to selecting individual folders and files to sync.

Have you tried the iCloud Drive feature? Not adding your Desktop and Documents folder to it but just the one folder you want to sync between Macs. It should fit into the free 5 GB of iCloud space that you have.

Hate contradict you but iCloud is the answer.