Blacks/dark colors being crushed/losing quality on export.

It is troubleshooting, at its most basic. You have a problem, "we" try to sort it out. That's what troubleshooting is.

And the problem you have is as I pretty much suspected, you are not setting up proper color management throughout your project. You don't have a clip selected, so I can't see what color space your clips are.

You don't have auto-detect log nor auto tonemapping on, but do have a sequence set to Rec.709. Your media isn't being "managed" into that color space at this time.

So ... what is your media color space? And what happens if you turn on both detect log and tonemapping?

Nope. Not being snarky, because I want to help, that's why I pop in here.

But from your post, there's nothing there for myself or anyone else to do anything with but guess. I prefer troubleshooting to guessing. But troubleshooting typically requires things like OS/CPU/RAM/GPU and for this, GPU driver. The media involved, shot by what, what format/codec.

Then also for color issues, screen grabs of the color management settings. Now all found in the Color Workspace/Lumetri panel/Settings tab. Do a screen grab of two, showing all settings there, and drag/drop onto the reply box so we don't have to 'open' an attached file to see them.

Then ... it's probably pretty quick to sort out.