Is there a better workaround or solution to boot a Proliant DL380 Gen8 from USB DVD drive than "Restore Default System Settings"?

Yes, there is a better approach to booting your server from physical media than resetting your BIOS to factory settings every time.

From HP's documentation... Title: How to Boot from External USB Optical Drive/USB Key?

HP ProLiant Gen8 Server Series:

USB Boot Support USB Boot Support controls whether the system boots from USB devices connected to the server. When disabled, this option also disables booting of iLO virtual media. Options include:

• Enabled (default)
• Disabled

Booting from USB device:

• Try with front and rear USB ports in the server.
• Booting into a External USB hard drive may not be supported.
• Avoid using USB hub while tring to boot.
• Make sure USB key has bootable files.