Hi @Tonyb,
You don't seem to have read this thread from start to finish. I can get into four boot modes.
1. UEFI boot mode Secure boot on - Standard setting with Legacy Options ROMs disabled - default.
2. UEFI boot mode Secure boot off with Legacy Options ROMs disabled.
3. UEFI boot mode Secure boot off with Legacy Options ROMs enabled.
4. Legacy Boot mode Secure Boot off with Legacy Options ROMs enabled.

Option 3 gets Legacy boot device options such as USB Storage Devices and also UEFI Boot options Onboard NIC IPV4 and IPV 6 but no Windows Boot Manager
Option 4 gets only Legacy Boot options such as USB Storage Devices but no UEFI boot options.

With both 3 and 4, the bootable pendrive starts booting but terminates with some error message. ( Read my post #8 in full ). I now believe that the Optane Memory comes in the way of booting from the pendrive.

Should I disable the Optane Memory in the UEFI Bios?

DELL has no clear cut documentation on it. I have read some documents which say "Warning: Do not disable Optane memory in BIOS unless you have first disabled the feature in the Intel RST application in Windows, else your drive and data will be inaccessible."
How do I disable Intel Optane? – CyberPowerPC Help Center

Now this is from DELL
"The option to Reset to non-Optane will clear all data including the operating system on the Hard Drive."
https://www.dell.com/support/article...-fails?lang=en ( Disabling Optane in the BIOS is equivalent to a failed Optane)

This is the only document on DELL website which then goes on to say these " Quick Tips have not been reviewed, validated or approved by Dell and should be used with appropriate caution."

I am still trying to understand the implications of disabling Optane in the BIOS. Till then I shall not experiment with it
Your self-assembled PC does not have Intel Optane Memory and not many people have a system with Optane memory. In Ten forums I know only one person who has a Dell Inspiron with Optane Memory - thomaseg1 and only one person who knows something about Optane Memory - Pejole2165. optimize drive / optane memory