It sounds like the cold-run condition is the only problem. It starts ok cold, but just won’t run well until the engine warms up, right? I tend to concur w/Tester, that symptom is consistent with the choke pull-off function. At least on my Ford truck’s carb, the carb supposed to cold-start with the choke plate fully closed, then immediately after the engine starts, the choke plate should open slightly, like 1/8 of an inch, open, but just barely. The choke pull off is done w/a vacuum operated gadget on my truck, and if the diaphragm that operates the vacuum motor ruptures, this symptom could develop.
One idea, surf around if there’s any kits available that allow you to turn this into a cable operated manual choke system. If the choke pull off diaphragm is split, you’d still have to fix that b/c it would create a vacuum leak.
Another idea, 1980’s Subaru carbs had a failure mode that was very difficult to diagnose. My parents had that problem, and they finally just sold their Subbie to the mechanic. As I understand the issue, it has to do with the electronic control of the Subbie carb. This design complication is used for emissions and drivability reasons, and affects how the carb is configured as a function of ambient and/or coolant temperature. There’s one poster here who has some experience with this problem as I recall, but I can’t remember who it is. Is it @ok4450 who has some 1980’s Subaru carb experience ? In any event, that’s what you need is to find somebody who has that type of experience. Or find a book or service manual that explains how it works.
One final idea – besides the possibility of a mistake in your recent carb rebuild — at some point a carb can become in-re-buildable. Worn out parts, bushings that leak air, etc. Maybe that has happened to yours. Maybe see if you can find a replacement carb.