These are excellent credentials, but it’s also good to get some experience.
For instance, a major skill to develop is to understand Korean sentence structure. Why? Because then you will be able to understand some sentences that apparently make no sense. You will know it’s literal translation, you will go back to the Korean style of saying things (no, you don’t need to actually learn the language!) and understand the meaning, so that you know how to change it. Become used to finding sentences with the verb at the very end and develop strategies to deal with them. Inverting them is the simplest solution but it doesn’t work all the time, because sometimes you have to match the surprise factor and the actors’ reactions.
Then it’s the style. Matching the style to the speaker and situation. Learning to recognize formality levels (again, you don’t need to learn Korean for that - and I’m only mentioning Korean because that’s what I have experience in)
Then of course it’s the customs, food items, place names, cultural references, ways of speech which some people translate literally and some not … If you don’t know about them, no matter how excellent your knowledge of English, you’re bound to make blunders.
And the formatting. And how to shorten a sub if needed without losing crucial information.
I could make more examples but my point is that even a real-life professional editor and proofreader would still need to work with an experienced Viki editor on a couple of projects before tackiling something on their own.
You may be interested in these two threads:
English Subtitle Editing
and
What do you find difficult when you edit and what is time-consuming