Short answer on the Subaru - needed a transmission and could not find one at any price. Did eventually find a decent one, but it was more dumb luck & word of mouth than anything.
I gotta say that when I started looking, I saw an alarming amount of "Bugeye for sale, Excellent condition, just needs transmission". To Subaru's credit, every other gasket/washer/seal I needed was still available, right up until I got to actually cracking the transmission itself open. So, can't find a transmission, and can't find the parts to fix one either. So, yeah, if you want to try & future-proof yours, consider keeping an eye out for transmissions.
Speaking of, your WRX sounds bad-ass. I did at one point (briefly) consider putting a tow hitch on mine and ditching the truck, but the only hitch I could find was a class 1. Is that what you are using, or did you find bigger, or simply fabricate your own?
Wait, the block from a 2019 STI fits the heads from a 2003 WRX? Okay, that's... you clearly know more about mixing/matching Subaru parts than I do. I knew the 2004 STI block matched, but that's a 1 year gap, you're talking about a 16 year gap. Wow. Perhaps I should be asking you if there's any other transmissions that drop right in? I'm aware that the later (~2008?) Legacy Spec B transmission is supposed to be a drop in - has the right gear ratios & connections for the halfshafts. But the clutch is different somehow and the pickups for the speedometer have changed from analog to digital. Still, supposedly it can be done. I didn't have any luck finding one of those transmissions either though...