If only you had checked this forum for past experiences of first timers much much earlier.
It does not follow that getting a PPL results in a huge boost to your attractiveness as cadet material.
How do you know that getting a PPL will, in your words, "convince them that I'm really interested in flying"?
The sequence of events (the rejections thus far, at the first interview stage, combined with the sudden and conveniently timed appearance of a PPL for the 3rd try at the first interview) will more likely show that you are trying to get in using devices you think will work, as opposed to proving a continued and lifelong interest in flying aeroplanes. It will stick out like a sore thumb that you look like someone trying to beat the system and you have to work even harder to overcome any prejudices that may have already formed.
The fact that you produce a PPL from Malaysia, at a convenient moment, at maybe half price, compounds this perception of merely trying to ace the selection process and not being the real you.
Sort of like trying to study for an exam using model answers only, and then showing the exam board how many books of ten-year series you studied, as opposed to demonstrating genuine subject knowledge and interest, don't you think?
Perhaps the harsh reality is that in spite of your self-claimed interest, whch has not been convincing so far, is that there are other factors which are not helpful to your potential of being a successful SIA cadet.
I suggest that you NOT get a PPL, from Malaysia or anywhere else.