I'm facing the same problem here. The winscp stops retrying sending the file after 15 seconds of a connection lost?
the "option reconnect 120" does not work in here.
After so manny years the problem remains?
What i ended up doing is 2 things, 1st change the timeout of connection to 6000, that's gonna be long enough to complete any task. you'll find this in the advanced settings of your connection's login profile.
2nd i started using the "File station" (web interface) that comes with my nas for big file transfers.
Use the right tools for the job you need to do.