Is it possible to download watchOS, but postpone the installation?
If so, how?
WatchOS can only install when the Watch is on its charger. If you download it and don’t go thru with the installation by removing the Watch from the charger, then the next time around the update routine just starts over again, downloads again, validates again. So, I think the answer to your question is “no”, and in any event there’s no time saved by deferring installation.