You are all correct except the second sentence. It says 오늘은 김치를 만들어요. This means that a dude named "Today" is making kimchi. So long story short, the word 오늘 is a subject in this sentence so you need to change it wouldn't be. Such as: 오늘은 김치를 만드는 날이에요 = Today is a day where (we) make kimchi. 오늘 김치를 만들거에요 = (We are) going to make kimchi today.

If you want to say 'Today I made Kimchi.', 오늘은 김치를 만들었어요 is correct. 오늘 김치를 만들었어요 is also correct. The difference between the 2 sentences is that you say the first sentence when you want to emphasize '오늘'. 어제는 불고기를 만들었어요. 그리고(and) 오늘은 김치를 만들었어요.