<< Solved >> Replaced Graphics Card, Now Doesn't Boot

[12sockmonkeys]

EDIT: I retried booting, hitting f12, selecting the Unibeast USB and it booted Clover (this one has the black background instead of the white one I get when I boot my main hard drive). Once in that Clover menu with the black background, I selected to boot my main hard drive and...it worked!

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UPDATE 4/20/19 --- THINGS ARE LOOKING UP --- I booted with the Unibeast High Sierra Install USB and did a Time Machine backup of my system from six weeks ago. This deleted the successful fresh install I'd just made, which lands me back to where I was at the beginning of this thread...but with two important changes:

First, I removed my XFX r9 280x GPU (which to me is now more-or-less a paperweight with cooling fans) and am running my motherboard's iGPU instead.

Second, when turning the computer on I'm able to hit f12 and boot from my Unibeast High Sierra Install USB's Clover menu, and from that boot menu (which has the black background) I'm able to select my HD, now restored from Time Machine, and it will successfully boot my old computer, now resembling the one I had six weeks ago.

Per Gigamaxx's suggestion, I ordered a new GPU: the XFX Radeon RX 570 RS Black Edition, which should be arriving tomorrow.

Without using the Clover boot menu from my USB (with the black background), I'm still unable to boot though.

 

[12sockmonkeys]

UPDATE 4/20/19 --- Problem solved. The XFX Radeon RX 570 RS Black Edition arrived, I put it into my computer and booted without the Unibeast High Sierra USB, without changing anything at all from my old build, and it just works.

The lesson here: My GPU died, I replaced with another that I thought was the same but the VBIOS is different. I bought a newer AMD 570, put that in my computer, it's like the problem was never there.