With the sharingan he would've known how no?
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I don't think it's a chakra reserve issue like people are saying. It was never implied to be incredibly chakra taxing. I think the reasoning for why it required three of Minato's body guards to be properly utilized, speaks more to the complexity of the jutsu more than anything. Plus you need to be a really talented sensor like Tobirama or Minato to use it, and Kakashi isn't that.
Agreed, I do think the overall challenge of the jutsu is complexity, not just chakra usage.
But I think usage might still have been a factor in Kakashi's case. By which I mean, the experimentation and training required to learn FTG might've been too chakra taxing for Kakashi to accomplish while he had the Sharingan sucking away at his reserves.
Maybe he would've learned it in a world where Minato lived to teach him and Kakashi never got that eye.