Tall lefties swinging it at pace is such a rare challenge, though. He can't just not play the shot because otherwise he'll be stood there looking silly when Starc bends one back and takes his off peg. The only comparable bowler Smith himself has faced is Amir, definitely not at his best last series, certainly not as quick as Starc. Compare that to Starc who, in between the dross, is super difficult to face right now. As you yourself have been saying when comparing the English to the Aussie bowlers, the extra pace changes everything and why the other lefties Smith's faced don't really compare.
That tension whether to play or leave, that's literally the tough bit and horribly magnified at 145Km/h vs 135Km/h. One thing that can save you is getting more forward, you negate the swing and it simplifies the decision whether to leave because if it's slightly short, it won't swing enough to come back so you can leave or if it's full, you can get right forward and negate the swing. But Starc being tall ****s with that too; he can throw it up there to swing it and it'll bounce so even if you cover the swing it can take your edge, especially since he more consistently hits the seam than, say, Boult.
Starc being a bit erratic is literally the only thing stopping him from being the most nightmarish quick to face in world cricket right now.