Yeah, but where though? Where can I find a Fallout 3 actual disc???

Which is new, and for PC? -- Anyway, I already rebuilt my game.

Took about 4 hours, but it's back. Woo hoo!
I did not need the FO3 Anniversary Patcher. What I did was started all over with the reinstall from GoG.
And then instead of copy/pasting stuff from the old Steam folder into the new GoG-based folder (really bad idea; what was I thinking?) I went to my Downloads folder. I've been in the habit of saving archives from the very beginning; a really good habit. Everything's organized. Went into the Downloads > Utilities folder. Started with FOSE.
My version of FOSE is really old; it's from 2016, and the newest version was released in 2022. However, I read somewhere that if I use the newer version then I'll have to make a bunch of upgrades, because my gamesaves are already extant. They are already based on older software. I would need the Anniversary Patcher if I went that route.
So that's gotta be why my game would instantly black-screen-on-launch.

Because that happened after I threw the Patcher in.
Anyway, I installed all the rest of the utilities: FO3 Edit, FOMM, and so on. Next, the Unofficial Patch. Finally, I moved to the mods, starting with Alternate Start folder. Everything was installed from scratch this time, straight from the archives themselves.
Enough talk. Time for some gaming. Miss Vicious, here I come.