UPDATE: After submitting the thought came to me that step 2 could be eliminated and you could make the thumb drive the bootable drive in step 4. Clear the bootable flag on the thumb drive first. Note that I haven't tested this modification but the original procedure was fully tested.
Summarizing the OP's issues:
1. I don't have a flash drive big enough.
2. I don't have burner and blanks with dual layer support.
3. Computer has no option in BIOS to boot from USB
4. Said computer's DVD drive can't read dual layer.
5. There is no OS to boot. No explorer. Hard drive crashed. Need clean install.
I was in essentially the same position when the 15 year-old HD in my legacy Dell GX520 crashed. I have a solution that respects all the OP's issues but does require purchasing (for < $5) or borrowing an 8GB USB thumb drive (not for boot).
- On another machine, use the media creation tool to create 20H2 installation media on the thumb drive.
- Use a gparted live-CD or another bootable disk partitioning utility to copy the partition on the thumb drive to the end of the wiped HD. You probably need to create a new MBR first. Do *not* mark the copied partition as bootable.
- Use a Win 7, 8, or 10 rescue CD to boot to a command prompt (do not use an old installation DVD).
- Make the copied partition the current drive. It may by D: or E: and will be identifiable by the appearance of setup.exe at the top level.
- Execute setup.exe. Use the custom partitioning option to make sure that the end partition does not get wiped away during installation.
- When the 20H2 installation completes you can delete the end partition and expand the windows partition if you wish or you can keep the end partition for possible restoration needs in the future.
Yes, I could do that, but I was not looking for a workaround. It's too complicated and would take much, much longer.
Extremely inconvenient. Who has so much time? Not supposed to be like that.
Should not be like that. Microsoft rather inconvenience the
end user than themselves.... Just saying.