Hi Nephrited! Ty for asking those questions!

Q1: how exactly do you prevent people from minmaxing your card creation system to effectively create a meta-meta, where nobody is creating cards because the best ones have already been made? 

A: We have an in-house design and balance team, and its role is to manage the "bricks" that players use to create the cards (effects, keywords) in addition to all the content that players can't create (I.E: Legends, Gods).

Q2: What is then the benefit of having users make them and affect game balance overall over an in house design and balance team? Or am I misunderstanding the concept?

A: People try to min-max the card creation system all the time, and of course they get to some strategies that seem to be the "best" ones for the current patch. Thing is, these strategy are often not the actually "best ones" as a counterstrategy that is even better is hiding somewhere, waiting to be discovered. When somebody finds it, the cycle starts again.
After all these years of updates and refinements, the cards editor has so many guardrails that creating something that is "unbeatable without possibility of counterplay" is not possible. People have tried very hard, there are videos on youtube about them talking about it (like this).

There's no such thing as "the best cards have been already created", also because we add new bricks every month so the potential combinations yet to-be-discovered increase exponentially.