1) Do you have your data properly backed up?
Note: No, a
single copy of your data is not a backup.
If disaster strikes where you live, are your backups redundant so you can recover?
2) With that in mind, consider your data syncing. Does your backup process let you do something like?
Machine 1 -> Backup -> Machine 2
Example: Machine 1 replicates to backup like a Cloud provider and machine 2 syncs from the cloud provider to the local machine.
If it does, you can setup certain folders to sync and you're basically done. If not, you might consider the value of redundant backups that allow you to do that. iCloud isn't the only Cloud. You can check /r/privacy's wiki on reddit and the
https://www.privacyguides.org/en/ website they link to for different private Cloud providers that keep your data encrypted end to end so nobody can access it except you.
3) If you have a locally hosted/offline redundant backup system, but still want offline sync, that starts with: What do you want to keep synced?
Do you just want to keep your Documents and Downloads folders synced? If it's something like that, many utilities can help like the command line rclone utility.