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I thought I might just quit that job altogether.
Literal
That work [topic-は] rather quit [volitional-よう] [quot-か] thought.
いっそ 'rather / might as well / if anything' is an adverb for committing to a drastic/simplifying course — often when a situation has become too complicated to handle piecemeal. やめようかと思う 'I thought I might quit' combines the volitional + か (tentative question) + と思う (reporting thought): 'I was thinking, should I just quit?' — a classic way to describe contemplating a decision without stating it as committed. The topic は contrastively sets 'that job' apart.