[12 piece bucket]

If the left shoulder has a distinct "up" component through impact and both-arms-straight might not fully occur until after the hands have passed the left shoulder (viewed side-on), then shouldn't the hands location begin to have a real and perceived "up" component passing the left shoulder?

(My full swing plane angle favors flat just a bit, on video it usually appears reasonably consistent and efficient. My current in-swing issues seem to be more about sequence, rhythm and balance ... elusive lag pressure, but I'm making progress. Still working on a chronic pre-swing problem of improperly aligning right of target, it's a perception-over-the-ball problem, that is all the AI I worked with two years ago left me with to work on, from behind looking down the target line his video of my swing showed the shaft getting on my right forearm soon and efficiently compared to most of his students. I fought a tendency to lay the shaft off just at the top, a compensation for a shoulder injury, it led to some inconsistencies, that glitch is much improved this past year.)

I think Homer would tell you that the FEELING would be DOWN even though the swing is going up because your arms are connected to your shoulders.

I'd say you'd want to feel down through the ball through low point through the aiming point and through both arm straight. At both arms straight generally your hands are coming back up plane but the feeling is down.