The setting _BESClient_Resource_StartupSleepSeconds allows to make the BESClient to sleep for some seconds (until 300, that is 5 minutes), after starting the BESClient service.
This way, the service will start but won’t perform any processing for the number of seconds set with _BESClient_Resource_StartupSleepSeconds.
During the Win10 upgrade, the delayed start will allow the OS to complete the post-reboot steps in the first minutes after rebooting and will prevent a second reboot forced by BESClient (and triggered by the registry settings configured by the OS at the first restart).
Based on that, the suggestion is to configure the setting (_BESClient_Resource_StartupSleepSeconds = 300) before applying the Win10 upgrade fixlet and run the fixlet as is, without changing anything in its actionscript.