If you are really serious about CSS, then I think you must quit your job qnd focus on tuition and CSS preparation since:
1)25k-30k Pay instead of 20k and 30k afterwards is a middle ground, given that job security issue is same for any private job, as in private tuition.

2)Being a tutor myself, this job gives you a lot of flexibility in timings and that is what will be mostly required by you for CSS preparation. Secondly, try to get tuition in nearby areas, thus saving even more time from travelling.

3)Tuition will also help you in revising and revisiting concepts of General Science, Pak Studies etc(if you provide tuition in those subjects)which will be an added bonus for CSS preparation.

4)In my personal experience, if you are lucky enough to land good and nearby tuition, then you may even get 2 to 3 days a week totally free by wisely scheduling your timings and this further reinforces the second point ie; flexibility of job timings.

So if and only if you are really interested and highly motivated for CSS preparation then go for the tuition option and take a break from private jobs for sometime, else, landing a private job is also not easy nowadays so it all depends on your motivation.

I, though, have just listed the benefits of the "Tutor Option" assuming that you are highly motivated for CSS Exams.

Best of Luck