Man with a rule like that's I'd max out Acrobatics and deal double damage by hopping on and off all my bigger-than-me foes.
Why not use Survival, which is the skill that governs riding skill?
Well, I dont think that this rule will be used very often, mabey never again.
I thought about survival too, but survival is more about "riding" an animal / anmimalplant. The player wanted to climb/jump on the Ksarik and stab it, so I will go with athletics.
Yes, skill checks will get so much higher than saving throws that matching them directly against each other does not make sense.
I also think that for proper difficulty scaling, DCs for skill checks based on enemy CR should probably use 1.5xCR, like feint and demoralize do.
The Skill Check is easier than the Save Check, thats right. But what if the enemy has arms (for example), then he will get a bouns. Its not that much about having a "hard" rule, we will use every session. My opening question was more about "How to handle Super Cool Cinema Actions" your Players want to do. If my Players start to abuse the improvised Rules I that I give them, then the checks will get harder and harder.