FreudShould interp always be an exact multiple of (1 / rate)? I remember reading (I think in Felik's config?) that 0.02 was a good value, which seems to contradict your and Comanglia's configs.
Also, is the tickrate in TF2 66 or 66.66...? (ie, is the minimum interp I should use 0.01515... or 0.015?) Again, I recall reading differing advice on this.
You can't set your interp lower than the lowest possible value, it get's automatically set to the lowest possible value for your current tickrate. https://i.imgur.com/CZ1tQak.png
The client will set it to interp_ratio/tickrate > 0.015152
Personally I've found that raising interp does very little other than delay projectiles, if other people have garbage computers/internet they're going to warp around and there's nothing you can do about it.