Waiting for all clients to be closed before activating the new service worker is not usable, as it will lead to user keeping the old code.
Drop the logic for now.

Fetch the list of precache url dynamically in order to decouple the worker code from the list of files.

Calculate dynamically the list of files from a python script, depending on the web site configuration.

Update the service worker as soon as the web site modification date changes.
This (I hope) will simplify ERP5 upgrade handling.