Sync two Macs over the internet?
stand alone app.
Not iCloud or iDrive or any other cloud method. Not NAS.
File/folder match.
2 M1 minis on Ventura in different locations.
Mac mini
Just a suggestion: Dropbox.com
It gets the job done in almost exactly the same way that iCloud works. They just don't call it a cloud.
You might explain to your client that the difference between iCloud and any other dedicated app for the task really just comes down to semantics. iCloud IS internet syncing and the client and host - data server, if you will - apps are well integrated by a company famous for its stance on privacy and security. No need to add a third-party sync app with a potential to be problematic.
Any 'internet sync' service will need a data host that makes files available to each computer. That host must have the capacity to store whatever amount of data is require by the client. That capacity is going to incur a cost to the client regardless of the hosting entity, whether it's 250 GB or 500 or 2 TB. This is exactly what iCloud does. Call it iCloud or call it Apple Internet Sync or call it Dropbox, it's the same service provision with the same features.
The only other option I see is the remote access that you say your client already has. That's an always on, or on-as-needed, dedicated internet connection to one of those computers acting as a server to host the client's files and allow direct remote access to those files by the other computer.
I agree w @BobTheFisherman and recommend iCloud.
To be blunt, I think you should educate your "self described tech dummy" client and alleviate their fear.
