2 options to try, firstly as an example of how to have it set up.
Synchronization Schedule is set to synchronize automatically at 9:00 PM, 1 synchronization per day.
Under Automatic Approvals – Advanced turn off automatically approve updates to the WSUS product itself and automatically approve new revisions of updates that are already approved.
Under Update Files and Languages - Update Files Tab tick "Store update files locally on this server" and untick "Download update files and to this server only when updates are approved".
The end result is that a synchronization will occur at about 9:00 PM each day, will pull down any new summary information and the associated update. This should get the end result you're looking for. Make sure you tweak Products and Classifications to only what you need.
Also you could try:
Create a batch file that starts the service "update services".
Create a batch file that stops the service "update services".
Schedule a task to run each batch file at the appropriate times.