Can't boot into RedHat 6.9. Excruciating install and boot times

I tried installing RHEL 6.9 Workstation on Dell Tower 7910 ( https://www.dell.com/en-us/work/shop/desktops-workstations/dell-precision-tower-7000-series-7910/spd/precision-t7910-workstation ). It was shipped with Ubuntu 14 pre-installed on UEFI. The client wanted RedHat 6.9 instead.

We tried installing through USB, but couldn't get past the part of the process where it asks to provide the partition where the install files are located. (We only saw a list of ~5 partitions, which we thought to belong to the pre-existing Ubuntu install).

We then installed RedHat 6.9 Workstation through DVD. We left it installing over night, and the installation was still running. I'm unsure if it went to sleep in the process, but it took many, many hours to install. At one point, it popped out the disc and asked for "Disc 1". I just put the disc that was already being used and clicked "OK". It accepted the disc and finished the installation.

After install, I get the Ubuntu boot screen, which takes ~15 minutes (or more) to complete the loading process (where the loading circle makes a complete circuit). Then, the circle disappears and it hangs on this screen for ~30 minutes+, which at that point, it decides to just restart:

If, I instead press "Escape" at this hanging boot screen, the following errors are thrown:

Here are the legacy boot options:

Here are the UEFI boot options:

Why is Ubuntu the only UEFI boot option? The shown error dialogue box is the error that is thrown when clicking the "Add boot option" button.

Here is Dell's "One-Time Boot Menu" (from pressing F12):

Now, when I try to boot into ubuntu (UEFI) from that menu, I get this error:

Does anyone have any advice? Should I try a USB install again?