With regard to your parents rejections, if your father was 51 the first time he applied for PR, then the rejection and subsequent rejection are to be expected as anecdotally, it is extremely rare for someone over the age of 45 to obtain PR. As he is not in an industry that is "critical" in the eyes of where the government wants to go, his expertise is not in short supply here. And there is the possibility that racial quotas may have contributed but in this case, I think it was more age & industry related.
However, in your case, I see a different scenario, which is probably a fair bit of quota issues but i also see another potential issue that was not apparent to me for a long time but as I'm now 74 it is staring me in the face, not so much PR as I am already that for over a quarter of a century. Picture this. You parents are/were rejected 4x thus far and mainly age related. For whatever the reason, if they give you PR it is a good possibility that you will ultimately gain SGC (if you want it). With that comes the possibility of applying for citizenship for your parents who are now senior citizens and there is a category for aged parents of SGCs (check out the ICA site). I've thought about using that vehicle myself as I have two children, both married with kids here now so they can act as my sponsor. While I already have PR makes it a shoe-in, but where the stumbling block for you, becoming a PR, it's on the pathway of also opening themselves up for collateral damages in increased healthcare costs as your parents are around my age, I'm guessing. By giving you PR they are opening up a potential can of worms as they have enough medical issues right now without arbitrarily potentially adding more.
I'm not saying this is fact, or fiction, but you have been here and educated well for 2 decades. It is a feasible position for the government to take. Giving you PR is the starting step in that direction as they obviously have had a reason to reject 4x and I'm guessing it's age related.
The other problem is one of NS. If your father has been on an EP for all those years, and you were in
International Schools, does this mean you have never been on a Dependent's Pass but on a Student's visa all those years? Also can I assume you didn't have any sort of grants to attend NUS? If your father did a number on you by keeping you off of a dependents pass so you could remain under the radar, it is going to be the straw that broke the camel's back as it were. You will not likely get PR because it will be potentially seen as you deliberately evaded NS and now want to eat you cake as well. Again, I am just pointing out 40 years of dealing with HR issues and my own in the process (I had two children born here with dual citizenship so had a lot of hurdles to deal with). I would still, in your case, apply for PR as it's is still worth a try but I wanted to try to help you see some of the pitfalls ahead. Good Luck.
SMS