Upload & Sell: Off
Even if you get a third party fix the mirror, which might be do-able, your shutter is highly likely to fail before long. Not because it is over 100,000 actuations, which is about the expected life of the shutter, but because it has been diagnosed by experts as nearing failure.
Yes, look into third party fixes.
But also see if somebody would buy your camera, as is for say $300 to 400 plus paying the repair bill, upon return from Canon.
As you know, the Classic is a great camera. You got a really good run with it.
Look at it this way: If you only shot, say, 10 K shots, then it might be worth, say, $900 as a low actuation 5D. Now it is worth about $400 or less due to "problems". So you got 90 K actuations on it for about $500, in terms of opportunity cost in today dollars.
Or another way of looking at it: Suppose you paid $2500 for it new or nearly new. That's 250,000 cents or 2.5 cents per shot.
Or another way to think of it: $400 repair gives you a camera good for another 100K clicks (no guarantees of course). Even if you only made 40K clicks with it, that would be a penny a click. Less because you can probably sell it a couple of years from now for $500.
So, yes, the repair is a good deal.
What you should be looking for is financing. (Metaphorically or actually)