What metrics or KPIs will you measure to adjust/nerf/buff the game?
Please provide reason/s for your points too.
Unfortunately, it wouldn’t all be up to me. I would have to be prepared to calculate and choose few indicators which closely align to the development strategy which was likely set by people directly above me in the company.
With that said, the easiest would be to simply track deck winrates, and perhaps calculate meta warp indexes for the top dogs. Now your goal is to balance decks, not individual cards, which means you have freedom to find those cards which impact the most decks that need to get nerfed/buffed.
The goal is to achieve balance with least possible amount of actions, as that lowers the chances of unforeseen circumstances emerging.
You should start with win rate and nothing else. That means win rate of a card when it exists inside a specific deck. That can expand to extreme complexity because the decks you can produce (and the combinations of their opponents and virtual tournaments) are a lot and other metrics can be derivative.
You should use simulation and formulation (think of SimCraft in WoW or the old Rawr) but the Devs are out of their depth for that.
To get a taste of what I mean look at the formulas of D0nkey for the impacts of cards (or if he removed them ask him at Discord).