Thanks, and don't worry, I interpret detailed responses as positive feedback.

whereas it's more of a smooth curve when I train at 5.

this is interesting and potentially important feedback. Quite a lot of routines based simply on what coaches saw as working seem to revolve around multiple sets of 5. People doing 4 rep routines is a lot less common so we have less data. The comparison is also not directly 5 reps vs 4 reps exactly since most newbie programs start at 5 and I am recommending starting at 10. I wouldn't hazard guesses for trained people, since they basically require periodization to advance optimally.

Would official lifting shoes be better?

Not for low bar. Shoes allow your shin to attain a greater angle without being limited by ankle dorsiflexion, which opens the hip more.

vegetarians or people who just don't eat much meat.

true, I use whey powder just to reach the 100g or so I want, so I should probably mention it.