The update to Sonoma from Ventura fails while downloading

I've had better luck from the Safe Mode because I evidently had some 3rd party software that interfered with the downloading and installing so try the following:

#1 - boot into Safe Mode according to Start up your Mac in safe mode - Apple Support.

NOTE: Safe Mode boot can take up to 3-5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

#2 - go to the System/Software Update preference pane and begin the download.

#3 - download and run the installer from the Safe Mode.

are you attempting the download over wi-fi? when possible, you should perform macOS downloads with a wired ethernet connection. if this is not possible, (mac has no ethernet port) you should move the mac as close as possible to the router, and temporarily remove your other devices from the wi-fi. you need the most bandwidth going to the mac as you can get while downloading a macOS. I would trash the current Sonoma installer (if there is one) and start over.