Yes, I agree that longer and lower is the way to go; it looks like you're sitting very 'squarely' on the saddle, which is restricting your ability to ability to get lower because your hip/back angle is so acute. I'd like to see you roll your hips forward more and get the bars out and down, things which should all be complementary.

Since you've got the other, longer stem and it's easy to do, I'd suggest starting there, by mounting it angled down (i.e. low/no rise position) on your current stack height (i.e. leave the spacers). If it feels like a step in the right direction, you can start moving the spacers, one at a time, up above the stem and see if you can go lower comfortably. Remember it will feel different, and you'll need to change how you sit on the saddle, but I think that's the way to go.

If the saddle feels too weird, get a different one; a new seat may ease the transition because you won't be as inclined to try to get back to your 'happy place' as you would on the old one.

Unfortunately, I don't think you can get stretched and more aero (i.e. into the drops) without going lower (because your butt is back on the saddle and low to the bars), and I don't see how you can get lower without rotating your hips forward, so you're going to be radically changing your position on the bike.

Try the stem swap and let us know what happens. Good luck!