Inspiron 7567, Used Legacy Boot, can't get UEFI w/Sec. Boot back

Specs:

• Laptop: Dell Inspiron 7567 (brand new)

• Processor: i5-7300HQ

• GPU: GeForce GTX 1050 Ti

• HDD: 256gb SSD

• OS: Windows 10 Home

• Drivers: Up to date

Summary: I got a new laptop, so I wanted to wipe the SSD and install a clean copy of Windows 10 Home. I used Parted Magic to 'wipe' the drive by issuing TRIM command from LiveUSB (created using Unetbootin). I then used the Microsoft Media Creation Tool to create a Live USB with Windows 10 Home.

I changed from UEFI with Secure Boot to Legacy Boot (BIOS) so I could change the boot order to load the Live USBs. The wipe of the drive and installation of clean Windows 10 Home went fine.

But now the UEFI choice is missing under the 'Repair' option (the normal way to get into UEFI Boot menu from a running Windows 10). So, I used F2 to get into BIOS and I manually change Legacy Boot back to UEFI with Secure Boot. However, upon reboot I'm greeted with the error "No bootable devices found." (The boot order is diskette drive > M.2 SATA SSD > USB > yada > yada > yada.)

So, I went back into BIOS (still Legacy Boot) and chose "Restore Settings." I tried both "BIOS Defaults" and "Factory Settings," hoping one of them would get me back to UEFI. No joy.

Questions:

• Can I get UEFI back? And, why should I bother? What's wrong with Legacy Boot?

• Can I get Secure Boot back? And, why should I bother? Will computer performance be better without it running? (Assuming I only get driver updates from Microsoft or directly from Intel and Nvidia.)