I want to say something like "After thinking about it, I decided not to take the class."
Would 考えてから、授業を取らないと決めました。be an accurate translation? Or is there a better one?
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After thinking about it, I decided not to take the class.
The most concise and natural way to say it in Japanese IMO is:
The key point is that やっぱり used with verbs meaning want/wish/decide etc., will become "on second thoughts". This definition is strangely missing from J-J dictionaries I've tried, but widely used in reality.
If you やっぱり want a more literal translation to your English, how about:
～と決めました would sound like you pledged to yourself never to break. It's not that serious situation, I guess. Also, 考えてから would be more like saying "I thought about it, and then —" in English.
"After thinking about it, I decided not to take the class." 「（色々）考えたのですが、その授業は取らないことに決めました。」There is no English word relevant to 「色々(in many ways)」 but in this situation I guess you'd like to express that you didn't decide instantly but you made well-considered decision or you don't intended to deny the value of the calss but just your circumstances have you to decide it.
Literary style: 「色々考えた末、その授業は取らないことに決めました。」
「考えてから、・・・」is a little bit unnatural but acceptable (Listener can fairly follow you).