I wish if there is possible to enhance to re-check the existing of files to skip or overwrite in the options script "by Time or/and size" on the local directory.
This hardly combines with the overall concept of how the synchronization works in WinSCP.
And it would not be safe anyway.
Another feature I hope for an option to increase the max number of transfer (winscp run under windows task scheduler) instead of currently one.
That's very difficult to implement.
But you can implement it on your own using PowerShell and WinSCP .NET assembly.
For parallel transfers:
For selection of files that need synchronization, see: