I've been testing a few things and found that two things help.

A full update seemed to fix the usbmount service failing to start. (Could be coincidental. I will advise if it turns out to be bad advice) sudo apt update sudo apt full-upgrade sudo reboot

UPDATE : Confirmed AOK. Occasionally I see the logs fly by with a red usbmount service failure, but within 2 seconds there is a matching line stating the usbmount service is up and running. All good.

It seems that the root cause of the extensive time delay to launch ES is ES searching the roms folders to see if there have been any new roms added. https://github.com/jrassa/EmulationStation/blob/master/GAMELISTS.md indicates that once I am happy with my roms I can change how ES is launched by using the switch --gamelist-only to skip automatic searching, and only display games defined in the system's gamelist.xml. Will update shortly after I've done some testing.

UPDATE: --gamelist-only switch works perfectly. Can also be set by entering RetroPie's Main menu > Other > and enabling Parse Gamelists Only. I can now have thousands of roms on the attached USB drive and ES loads in a few seconds. :-)

GameBoy Themed Splash Screens