Heretic of The Seventh Circle
It couldn't stack them, as I said in the OP.Not just divine favour, but also Holy Weapon, Hunters Mark (Veng), Bless, Elemental weapon and Spirit guardians to name a few.
The Paladin is a top tier damage dealer as is. Letting it stack Smite spells on top of any of those spells AND on top of Divine Smite is a bad idea for mine.
And if your players would never cast them if they exclude the use of Divine Smite, then Divine Smite is too powerful.
Now, IME, most players don't so wildly overvalue a few points of increased average damage that they are reluctant to use the smite spells, so I see no need to nerf DS, but obviously a lot of other tables do need to, by the sounds of it.
But you can already stack two of three. Adding the third increases nova, but nova is often wasted past a certain point, and comes at a cost.