How much lore is there about the Paruns who we haven't seen on a card?
Not a whole lot. Usually, we only know their names.
We know nothing about Simic other than their name.
We don’t even know the name of the Orzhov Parun.
All we know about Azor I is that he’s not originally from Ravnica (which doesn’t mean he’s a planeswalker, Ravnica is what’s called a Seeded Plane, meaning planeswalkers brought many creatures there) and he created the Dragon’s Maze and wrote the Guildpact.
Cisarzim (of the Gruul)’s torso was part of Svogthos’ (of the Golgari) necromantic body (Svogthir is represented by the animated Svogthos, the Restless Tomb).
Mat’Selesnya is an elemental that inhabits Vitu-Ghazi.