Thanks, Martin
I had to make a decision about which FTP server to use, and obviously S3 is not suitable for me, because using -criteria=size is not feasible in my case. I will continue with FTP on Synology.

Afaik, S3 protocol does not support preserving remote file timestamps.
(Definitely, WinSCP does not preserve the timestamps when uploading to S3).
So the -preservetime has no effect and indeed, your command would result in all files being uploaded every time, as all remote files have different timestamps (times of the upload). So WinSCP sees all of them as modified.
You can remove "time" from synchronization criteria. And you might want to add "size":

https://winscp.net/eng/docs/scriptcommand_synchronize#criteria
Though obviously, modified files, that do not change their size, won't be synchronized.

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It might be useful to support checksum-based synchronization with S3 protocol. Though it has limitations on AWS side for large files. So it may not work in the end.
Issue 2295 – Checksum support for S3 protocol

Hello

I use this script to connect to Google Cloud:

Everything is fine, except that it doesn't sync, rather it copies all the content, even if the files are identical.
I've also tried it from PowerShell and the result is the same.
If I use the FTP on my synology, the sync works correctly.

What am I doing wrong?