Career Suggestion: Should I try to improve my manuscript or try to increase paper count after submitting PhD thesis
If the question is whether to emphasize quality or quantity of publications, the answer is "both." But if you have to pick one, go for quality. People understand that peer review takes a long time, so it will not necessarily be held against you if your papers are still under review when you're applying for jobs. But it will be held against you if the papers are substandard in quality, or if they're published in mediocre journals.


  Should I submit the unpublished works in tier 1 journals and try to improve the quality of the manuscripts (with the reviewer's suggestions)


You should first of all improve the manuscripts as much as you can on your own, and with your advisor's help. Then submit to appropriate journals. Manuscripts do sometimes improve with referee comments, but just as often, a referee will reject a paper without detailed feedback (especially if the paper is clearly not good enough). Attempting to treat peer review as a free editing/consulting service would be a waste of the editor's and referee's time.