Pseudodragon poison DC is CON based. Says so in creature description.
As for other sugestions, have you thought about Hellspawned Grace invocation? It isn't great, but it fits the theme.
"The save DC is Constitution-based and includes a +2 racial bonus."
I do believe my eyes have always glazed over this portion of entry before for any monster, and it never occurred to me that boosting CON boosts DC (which now is obvious). Thanks for teaching me something new!
As for Hellspawned Grace, I did weigh it, especially with the hellcat title. Looking at the monster entry, Puck already has most of the qualities (Telepathy, fire resistance, scent, some DR) and while I passed over invisibility in his invocation list, by the time he's high enough level, enough monsters can see invisibility anyway.
The only think becoming a Hellcat would do would grant melee ability, which Puck is wired against. He's a ranged attacker (Eldritch Blasts, Tenacious Plague), enemy debuffer (Utterdark Blast, Devour Magic, Word of Changing, Painful Slumber of the Ages). He doesn't have the AC or the HP to go toe to toe.
