Hello,
After some testing, I wanted to add some precisions if someone else wonder the same question.
It is not really the last class you took a level in that matters, it is the last class you added to the character.
If you start with sorcerer at level 1, take 2 levels in cleric for Destructive Warth and then finish leveling in sorcerer, Markoheshkir will still use Wisdom as its spellcasting modifier
If you want to use Charisma you have to start with Cleric at level 1.
It is not really ideal since you loose constitution saving throw proficiency and have a less interesting choice of skills in my opinion.
A workaround you can use is adding a level in a different charisma-based spellcaster later, like a level in Bard.
Another note: sometimes it is necessary to unequip/re-equip the staff to update the spell save DC. I think it is fixed the moment you cast Kereska's favour, not 100% sure though