@idosubaru My man!! You sound like you've been around the block a good bit with this stuff, thanks for the reply.
Two buddies and I actually spent the first half of this year's weekends swapping a JDM EJ205 into my car. The factory USDM motor blew a head gasket finally at 256,000 miles and I thought a JDM swap was the cheaper/easier alternative... boy was that stupid idea #1. Everything went fine except one glaring issue that we installed the USDM intake cam gears slightly wrong so the car idles and runs terribly under 3,000 RPM... I wonder if maybe the aggressive jerky idling abused the already aging transmission then? The timing being off causing the motor to run like spoob is mostly presumptive though.
I only point blame onto the radiator because of course we removed the radiator for the JDM swap, lost some fluid, then when we completed the swap and reinstalled the rad, we broke the coolant nipple on the top passenger-side corner, so I bought a new OEM radiator and through replacing that we lost more fluid. Like you said though, couldn't have been more than a pint! He convinced me it was the loss of fluid's fault because his girlfriend's much newer Crosstrek apparently went bad due to losing a bit of fluid or something like that...
So probably my best bet is to just find the exact same transmission from an Impreza then? I definitely don't wanna continue complicating this car trying to match up the rear diff to the Legacy's trans. Hoping that along with fixing the JDM engine's timing is all that's needed to get this car right again!