Would you recommend proofreading as a way to earn a second income?

If so, I would be glad to hear some recommendations for accredited courses - I have already looked up a few including a distance learning course which is about £180 - is it worth it/necessary, when my grammatical accuracy, spelling etc., is pretty good anyway? I believe that companies always get at least 2 people to proofread something in case one inadvertently misses an error. I know the main focus of a course is to learn the correct ways to mark the texts, whether for editor, printer etc. Please share any thoughts!

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proofreading services